The role of lipid transfer proteins in plasma lipoprotein metabolism

Browning deficiency and low mobilization of fatty acids in gonadal white adipose tissue leads to decreased cold-tolerance of transglutaminase 2 knock-out mice

During cold-exposure 'beige' adipocytes with increased mitochondrial content are activated in white adipose tissue (WAT). These cells, similarly to brown adipose tissue (BAT), dissipate stored chemical energy in the form of heat with the help of uncoupling protein 1 (UCP1). We investigated the effect of tissue transglutaminase (TG2) ablation on the function of ATs in mice. Although TG2^+/+ and TG2^-/- mice had the same amount of WAT and BAT, we found that TG2^+/+ animals could tolerate acute cold exposure for 4h, whereas TG2^-/- mice only for 3h. Both TG2^-/- and TG2^+/+ animals used up half of the triacylglycerol content of subcutaneous WAT (SCAT) after 3h treatment; however, TG2^-/- mice still possessed markedly whiter and higher amount of gonadal WAT (GONAT) as reflected in the larger size of adipocytes and lower free fatty acid levels in serum. Furthermore, lower expression of 'beige' marker genes such as UCP1, TBX1 and TNFRFS9 was observed after cold exposure in GONAT of TG2^-/- mice, paralleled with a lower level of UCP1 protein and a decreased mitochondrial content. The detected changes in gene expression of Resistin and Adiponectin did not provoke glucose intolerance in the investigated TG2^-/- mice, and TG2 deletion did not influence adrenaline, noradrenaline, glucagon and insulin production. Our data suggest that TG2 has a tissue-specific role in GONAT function and browning, which becomes apparent under acute cold exposure.

HDL-subpopulation patterns in response to reductions in dietary total and saturated fat intakes in healthy subjects

BACKGROUND

Little information is available about HDL subpopulations during dietary changes.

OBJECTIVE

The objective was to investigate the effect of reductions in total and saturated fat intakes on HDL subpopulations.

DESIGN

Multiracial, young and elderly men and women (n = 103) participating in the double-blind, randomized DELTA (Dietary Effects on Lipoproteins and Thrombogenic Activities) Study consumed 3 different diets, each for 8 wk: an average American diet (AAD: 34.3% total fat,15.0% saturated fat), the American Heart Association Step I diet (28.6% total fat, 9.0% saturated fat), and a diet low in saturated fat (25.3% total fat, 6.1% saturated fat).

RESULTS

HDL(2)-cholesterol concentrations, by differential precipitation, decreased (P < 0.001) in a stepwise fashion after the reduction of total and saturated fat: 0.58 +/- 0.21, 0.53 +/- 0.19, and 0.48 +/- 0.18 mmol/L with the AAD, Step I, and low-fat diets, respectively. HDL(3) cholesterol decreased (P < 0.01) less: 0.76 +/- 0.13, 0.73 +/- 0.12, and 0.72 +/- 0.11 mmol/L with the AAD, Step I, and low-fat diets, respectively. As measured by nondenaturing gradient gel electrophoresis, the larger-size HDL(2b) subpopulation decreased with the reduction in dietary fat, and a corresponding relative increase was seen for the smaller-sized HDL(3a, 3b), and (3c) subpopulations (P < 0.01). HDL(2)-cholesterol concentrations correlated negatively with serum triacylglycerol concentrations on all 3 diets: r = -0.46, -0.37, and -0.45 with the AAD, Step I, and low-fat diets, respectively (P < 0.0001). A similar negative correlation was seen for HDL(2b), whereas HDL(3a, 3b), and (3c) correlated positively with triacylglycerol concentrations. Diet-induced changes in serum triacylglycerol were negatively correlated with changes in HDL(2) and HDL(2b) cholesterol.

CONCLUSIONS

A reduction in dietary total and saturated fat decreased both large (HDL(2) and HDL(2b)) and small, dense HDL subpopulations, although decreases in HDL(2) and HDL(2b) were most pronounced.

Remodeling of the HDL in NIDDM: a fundamental role for cholesteryl ester transfer protein

When the Ay gene is expressed in KK mice, the yellow offspring (KKAy mice) become obese, insulin resistant, hyperglycemic, and severely hypertriglyceridemic, yet they maintain extraordinarily high plasma high-density lipoprotein (HDL) levels. Mice lack the ability to redistribute neutral lipids among circulating lipoproteins, a process catalyzed in humans by cholesteryl ester transfer protein (CETP). To test the hypothesis that it is the absence of CETP that allows these hypertriglyceridemic mice to maintain high plasma HDL levels, simian CETP was expressed in the KKAy mouse. The KKAy-CETP mice retained the principal characteristics of KKAy mice except that their plasma HDL levels were reduced (from 159 +/- 25 to 25 +/- 6 mg/dl) and their free apolipoprotein A-I concentrations increased (from 7 +/- 3 to 22 +/- 6 mg/dl). These changes appeared to result from a CETP-induced enrichment of the HDL with triglyceride (from 6 +/- 2 to 60 +/- 18 mol of triglyceride/mol of HDL), an alteration that renders HDL susceptible to destruction by lipases. These data support the premise that CETP-mediated remodeling of the HDL is responsible for the low levels of that lipoprotein that accompany hypertriglyceridemic non-insulin-dependent diabetes mellitus.

High-density lipoprotein: relations to metabolic parameters and severity of coronary artery disease

The regulation of plasma high-density lipoprotein (HDL) cholesterol level by the joint influence of plasma lipoprotein lipids, lipoprotein lipase (LPL), hepatic lipase (HL), cholesteryl ester transfer protein (CETP), oral glucose tolerance, and postload plasma insulin and proinsulin levels was investigated in young postinfarction patients and healthy population-based control subjects. In addition, the association between HDL cholesterol and the number and severity of coronary stenoses previously reported in this cohort of young postinfarction patients was further investigated by analyzing the determinants and angiographic relations of HDL subclasses measured by gradient gel electrophoresis. The following parameters showed significant univariate relations with HDL cholesterol level in the patient group: very-low-density lipoprotein (VLDL) cholesterol and triglyceride, low-density lipoprotein (LDL) triglyceride, and postload plasma insulin concentrations, preheparin plasma LPL mass, and postheparin plasma HL activity. In the control group, significant correlations with HDL cholesterol concentration in addition to those noted among the patients were found for body mass index (BMI), LDL cholesterol level, postload plasma intact proinsulin concentration, and LPL activity in postheparin plasma. In contrast to the patients, no significant relations were noted for postload plasma insulin level and preheparin plasma LPL mass. Multiple stepwise regression analysis showed that 42% of the variability of HDL cholesterol in the patients could be accounted for by VLDL cholesterol concentration (29%), LDL triglyceride level (7%), and postheparin plasma HL activity (8%), whereas the corresponding figure in controls was 35% (VLDL cholesterol concentration [9%] and postheparin plasma HL activity [26%]. The strength of the relationships of HDL cholesterol and HDL subclasses to the coronary stenosis score was similar and statistically significant (r = .25 to .36). When the metabolic parameters that correlated with HDL cholesterol and HDL subclass concentrations in univariate analysis were used as covariates, all relations to the coronary stenosis score disappeared. This clearly indicates that the influence of triglyceride-rich lipoproteins and lipolytic enzymes needs to be considered when assessing the association between HDL cholesterol and coronary artery disease (CAD).

Decreased affinity of low density lipoprotein (LDL) particles for LDL receptors in patients with cholesteryl ester transfer protein deficiency

We have reported that the disorder of lipoprotein metabolism in hyperalphalipoproteinaemic patients with a deficiency of cholesteryl ester transfer protein (CETP) is characterized by the polydisperse low density lipoprotein (LDL) particles and the accumulation of cholesteryl ester (CE) in high density lipoprotein (HDL) particles, forming cholesterol-induced HDL (HDLc)-like particles. In the present study we have investigated the interaction of these abnormal LDL with LDL receptors of normal human fibroblasts. Since the ultracentrifugally separated LDL fraction (1.019 < d < 1.063 g mL-1) from the CETP-deficient patients contained HDLc-like particles, these particles were removed by anti-apolipoprotein (apo) A-I immunoaffinity column chromatography. The lipoproteins eluted in the unbound fraction of this column did not contain apo A-I, so this fraction was considered to be authentic LDL. The authentic LDL of the patients were deficient in CE and rich in triglycerides and apo B. The authentic LDL itself showed polydispersity, ranging in size from 23 nm to 30 nm. The affinity of these abnormal LDL particles for LDL receptors was analysed by a competitive assay in which cold LDL from the patients or control compete with 125I-labelled LDL for fibroblast LDL receptors. The concentration of LDL particles at which 50% of 125I-labelled normal LDL was replaced was two to three times higher for the patients than for the normal control. Therefore, the affinity of patient LDL was thought to be reduced compared to that of control LDL. These results demonstrate that CETP may play an important role in making LDL particles homogeneous and rich in CE.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of human sperm capacitation by purified lipid transfer protein

Lipid Transfer Protein I is recognized as a key component involved in high density lipoprotein metabolism. We have been studying lipid transfer in relation to sperm capacitation, a complex series of cell surface events required for the acrosome reaction and fertilization. We have previously shown that Lipid Transfer Protein I is present and active in the female reproductive tract. In the present study, we show that purified Lipid Transfer Protein I directly stimulates human sperm capacitation, but not the acrosome reaction, in the absence of other biological effectors. These results provide strong evidence for a novel role for Lipid Transfer Protein I and reveal, for the first time, a potent activator of capacitation, prior to the acrosome reaction.

Frequency of exon 15 missense mutation (442D:G) in cholesteryl ester transfer protein gene in hyperalphalipoproteinemic Japanese subjects

Cholesteryl ester transfer protein (CETP) transfers cholesteryl ester from high density lipoprotein (HDL) to apo B-containing lipoproteins. The hyperalphalipoproteinemia caused by CETP deficiency is fairly common in Japan and one of the most common mutations in the CETP gene is the splicing defect of the intron 14, the allelic frequency of which has been shown to be 0.0049 in the Japanese general population. Recently, we have reported a missense mutation in exon 15 of the CETP gene (442D:G), showing a dominant effect on the CETP activity and HDL-cholesterol level. In the current study, we determined the frequency of this new mutation in Japanese hyperalphalipoproteinemic (HDL-cholesterol > or = 100 mg/dl) subjects. A rapid and easy screening method for this new mutation was developed using a polymerase chain reaction (PCR)-mediated site-directed mutagenesis. Among 117 Japanese hyperalphalipoproteinemic subjects (HDL-cholesterol; 116.7 +/- 16.5 mg/dl, mean +/- S.D.) without the intron 14 splice defect, three homozygotes (2.5%) and 34 heterozygotes (29.1%) were found to have the 442D:G mutation. The relative allelic frequency of this mutation was calculated to be 0.17. One of the homozygotes for the 442D:G mutation was the patient previously described by us as having hyperalphalipoproteinemia with corneal opacity and coronary heart disease. This was the first reported subject homozygous for the CETP deficiency who also demonstrated atherosclerotic symptoms. In homozygous subjects, CETP activity ranged from 37% to 62% of the normal value, which was consistent with the results obtained from the transient expression experiment previously reported; however, the specific activity of CETP was not as low as expected.(ABSTRACT TRUNCATED AT 250 WORDS)

Greater selective uptake by Hep G2 cells of high-density lipoprotein cholesteryl ester hydroperoxides than of unoxidized cholesteryl esters

We have observed recently that high-density lipoproteins (HDL) are the predominant carriers of cholesteryl ester hydroperoxides (CEOOH), the major class of lipid hydroperoxides detectable at nanomolar concentrations in the plasma of healthy fasting humans. The present study investigates the effect of such very low levels of CEOOH in apolipoprotein E-free HDL3 on lipoprotein particle metabolism and 'selective uptake' of its CE by human Hep G2 cells. Minimal oxidation with aqueous peroxyl radicals had a negligible effect on the binding, internalization and degradation of 125I-labelled HDL3. In contrast, with an increasing degree of radical-mediated oxidation of labelled HDL3, [3H]cholesteryl linoleate ([3H]Ch18:2) was taken up at an increasingly greater rate than were 125I-apoproteins. When [3H]cholesteryl linoleate hydroperoxide ([3H]Ch18:2-OOH was incorporated into unoxidized HDL3 by exchange from donor liposomes, it was taken up at a more than 8-fold higher rate than was incorporated [3H]Ch18:2. The same degree of preferential uptake of oxidized CE was observed when HDL3 was used that was doubly labelled with [3H]Ch18:2-OOH and cholesteryl [14C]oleate ([14C]Ch18:1). In both situations, uptake of [3H]Ch18:2-OOH exceeded that of 125I-apolipoprotein A-I some 40-fold. This increased selective uptake of [3H]Ch18:2-OOH from very mildly oxidized HDL3 was accompanied by a parallel increase in the intracellular levels of labelled free cholesterol. In contrast, lipid hydroperoxides were not detectable within Hep G2 cells, suggesting efficient detoxification of CEOOH by these cells. Neither the increased selective uptake of Ch18:2-OOHs nor the levels of intracellular free cholesterol were influenced by the presence of 50 microM chloroquine, suggesting extralysosomal hydrolysis of oxidized CEs. These results show that the selective uptake of HDL CEOOH by Hep G2 cells is more efficient than that of unoxidized CE, and support a protective role for rapid selective uptake in the removal of circulating HDL CEOOH.

Exercise training decreases plasma cholesteryl ester transfer protein

To assess the effect of exercise on the plasma concentration of cholesterol ester transfer protein (CETP) and its possible influence in mediating the exercise-associated redistribution of cholesterol among plasma lipoproteins, we measured plasma CETP in 57 healthy normolipidemic men and women before and after 9 to 12 months of exercise training. The training protocol resulted in significant changes in VO2max (mean +/- SD, +5.3 +/- 3.5 mL.kg-1 x min-1), body weight (-2.5 +/- 3.5 kg), plasma triglycerides (-25.7 +/- 36.3 mg/dL), high-density lipoprotein cholesterol (HDL-C) (+2.6 +/- 6.2 mg/dL), and ratios of total cholesterol to HDL-C (-0.30 +/- 0.52) and low-density lipoprotein cholesterol (LDL-C) to HDL-C (-0.18 +/- 0.45) (all P < or = .05) but no change in lipoprotein(a). CETP concentration (in milligrams per liter) fell significantly in response to training in both men (n = 28, 2.47 +/- 0.66 to 2.12 +/- 0.43; % delta = 14.2%; P < .005) and women (n = 29, 2.72 +/- 1.01 to 2.36 +/- 0.76; % delta = 13.2%; P < .047). The CETP change was observed both in subjects who lost weight (n = 28, delta mean weight = -5.0 kg; delta CETP = -0.42 +/- 0.79; % delta = 15.4%; P < .009) and in those who were weight stable (n = 29, delta mean weight = -0.12 kg; delta CETP = -0.29 +/- 0.78; % delta = 10.4%; P < .055).(ABSTRACT TRUNCATED AT 250 WORDS)

Marathon runners presented lower serum cholesteryl ester transfer activity than sedentary subjects

Acute exercise promotes raised HDL cholesterol concentrations by lipolysis stimulation, but this effect is insufficient to explain the more permanent HDL increases seen during regular exercise. During training periods in a group of marathon runners, we measured lipid transfer protein I (LTP-I)-mediated cholesteryl ester transfer activity (CETA) and its relationship to their HDL concentrations. Runners of both sexes showed significantly lower CETA values than those of sedentary controls. Male runners also had significantly lower serum concentrations of triglyceride, VLDL cholesterol and apolipoprotein B, and significantly higher concentrations of HDL cholesterol and apolipoprotein A-I than male controls. Results indicate that regular practice of aerobic exercise promotes modifications of lipoprotein metabolism related not only to lipolysis, but also to lower CETA. Such modifications are associated with reduced risk of atherosclerosis.

Lipoprotein profile characterization of the KKA(y) mouse, a rodent model of type II diabetes, before and after treatment with the insulin-sensitizing agent pioglitazone

The purpose of this study was to characterize the lipoprotein profile in the KKA(y) mouse, a rodent model of type II diabetes, before and after treatment with the insulin-sensitizing drug pioglitazone. Analysis of the plasma from untreated KKA(y) mice showed that they were severely hyperglycemic, severely hypertriglyceridemic, and moderately hypercholesterolemic. Agarose column chromatographic analysis showed that essentially all of the triglyceride eluted with very low density lipoprotein, and the majority of the cholesterol eluted with high density lipoprotein. Thus, both the very low density lipoprotein and high density lipoprotein levels were markedly elevated in KKA(y) mice. Analysis of the lipoproteins by agarose electrophoresis-immunoblotting showed that apoprotein A-I and apoprotein B had aberrant electrophoretic behavior, typical of apoproteins that have been modified by nonenzymatic glycosylation. Treatment of KKA(y) mice with pioglitazone for 8 days caused a marked reduction in blood glucose and plasma triglyceride concentrations but had no significant effect on plasma cholesterol concentration or distribution. The aberrant electrophoretic behavior of the apoproteins was corrected to normal by drug treatment. These data show that the KKAy mouse has a severe dyslipoproteinemia that is probably secondary to its insulin resistance, but that its lipoprotein profile differs significantly from that of the insulin-resistant human in that the majority of the plasma cholesterol is carried in high density lipoprotein, and those high density lipoprotein levels are very high.

The regulation of hepatic lipase and cholesteryl ester transfer protein activity in the cholesterol fed rabbit

Hepatic lipase (HL) and cholesteryl ester transfer protein (CETP) activities are both increased in the rabbit by cholesterol feeding. The in vivo regulation of HL and CETP were explored by examining changes in specific steady-state mRNA levels upon cholesterol feeding. On feeding rabbits cholesterol, HL activity increased 3-fold after 2 days and remained at 2.6-times the control value at 28 days. Specific rabbit HL mRNA levels were assessed by dot blot analysis of liver poly (A)+ RNA hybridized with the human HL cDNA. No significant changes in liver HL mRNA accompanied the increase in activity seen at days 2 and 7. At day 28 a modest rise of 46% was observed. A significant rise in CETP activity, evident 7 days after the commencement of cholesterol feeding, was maintained until day 28 when it was 2.4-times the control value. Using the human CETP cDNA as probe, rabbit liver CETP mRNA was also found to increase by day 7, rising to 3.7-times control by day 28. The strong temporal relationship between the rise in CETP activity and mRNA (r = 0.55, P = 0.02) suggests that the regulation of CETP may be primarily effected by the levels of specific mRNA. In contrast, the discordance between levels of lipase activity and mRNA suggests that post-transcriptional events may be more important in the regulation of HL in the cholesterol fed rabbit.

Detection of two species of low density lipoprotein particles in cholesteryl ester transfer protein deficiency

By equilibrium density gradient ultracentrifugation, we analyzed the chemical composition and particle size of low density lipoproteins (LDLs) in 16 subfractions separated from the LDL fractions (1.019 less than d less than 1.063 g/ml) of two hyperalphalipoproteinemic patients who had a deficiency of cholesteryl ester transfer protein (CETP). The LDLs of these patients comprised a group of heterogeneous lipoprotein particles distributed almost equally in a wide density range from d = 1.025 g/ml to d = 1.053 g/ml, whereas LDLs from normal controls were a homogeneous group of lipoprotein particles distributed in a narrow density range from d = 1.030 g/ml to d = 1.046 g/ml. The LDL in each subfraction derived from the patients' plasma samples was poor in cholesteryl ester and rich in triglycerides and apolipoproteins. Each subfraction of normal control LDL contained only one species of homogeneous LDL particles, which progressively decreased in size with an increase in the density of the fraction. In contrast, each subfraction of patient LDL contained two species of LDL particles: smaller LDLs existed, in addition to those that were found to be identical to the normal control LDL particles observed in the corresponding subfractions. The intermediate density lipoproteins of the two patients were also composed of two species of lipoproteins. From these results, we speculate that two metabolic pathways may exist in the LDL formation process. In this process, the transfer of cholesteryl ester from high density lipoproteins by CETP may convert the smaller lipoprotein particles to the larger ones, forming the homogeneous LDL species.

Esterification of dehydroepiandrosterone by human plasma HDL

Evidence for metabolic esterification of dehydroepiandrosterone (DHEA) in human blood plasma, identification of the active lipoprotein (LP) subclass involved, namely HDL3, as well as positive identification of the long-chain fatty acid esters of DHEA formed as incubation products is presented. The esterification reaction of DHEA and subsequent transfer and transport of DHEA esters in human plasma appears to proceed in a manner similar to that of cholesterol. The experiments presented serve as a model predicting similar metabolic transformations during HDL3 interactions with other steroid hormones that have the delta 5-3 beta-hydroxy steroid ring structure and exhibit nonequilibrium associations with HDL. These observations imply that significant quantities of DHEA, particularly in the conjugated ester form, can enter cells via the membrane receptor-mediated pathways of LP internalization.

Phosphatidylcholine metabolism after transfer from lipid emulsions injected intravenously in rats. Implications for high-density lipoprotein metabolism

When injected intravenously in rats, emulsion models of triacylglycerol-rich lipoproteins were metabolized like natural lipoproteins and during the hydrolysis of emulsion triacylglycerols, a large fraction of the emulsion phosphatidylcholine was transferred to the plasma high-density lipoproteins. The removal from plasma of emulsion phosphatidylcholine was followed for 2 h in unanaesthetized rats. The half-lives for removal of phospholipid after injection of emulsions stabilized with dioleoylphosphatidylcholine or 1-palmitoyl-2-oleolyphosphatidylcholine were 58-63 min when traced with isologous label. In comparison, the published half-lives of HDL mixed phospholipids in rats are approx. 40 min, indicating that much of the clearance of the emulsion phospholipid could be accounted for by HDL catabolism. Measured LCAT activity was sufficient to account for not more than 2% of the catabolism of the HDL phospholipids labelled by this physiological procedure. Removal from plasma of label was more rapid when the same emulsions were labelled with tracer amounts of the heterologous dipalmitoylphosphatidylcholine, showing that individual phosphatidylcholine species were handled distinctly even when present only in tracer amounts in a bulk of another phosphatidylcholine differing in acyl chains.

High-density lipoprotein turnover

High-density lipoprotein (HDL) metabolism has been reviewed from information derived from turnover studies in humans. The two major HDL apoproteins AI and AII have different removal rates, reflecting the faster catabolism of HDL2 than of HDL3. This is caused by the continual cycle of formation of HDL2 from HDL3 and its reversion to HDL3, in response to the need to transport cholesterol and other lipids from extrahepatic cells and catabolized triglyceride-rich lipoproteins. The conversion of HDL2 to HDL3 is mediated through a hepatic lipase. Because this lipase is inhibited by estrogen and stimulated by androgens, women have higher HDL2 levels than men. The synthesis of apoproteins AI and AII is also higher in women than in men. Nutrition also influences HDL turnover. Carbohydrates increase AI and HDL2 removal, whereas polyunsaturated fatty acids inhibit synthesis. Vegetarians show high HDL removal rates. Thus low-fat, low-cholesterol diets generally lead to lower HDL levels. Disorders that alter HDL composition (such as alcoholic liver disease or Tangier disease) accelerate HDL removal. Other HDL proteins such as apoproteins C and E show faster turnover rates than AI and AII, since the former exchange with triglyceride-rich lipoproteins and participate in their catabolism. Diminished exchange of apoprotein C from HDL to chylomicrons may be responsible for the diminished catabolism of these particles in type V hyperlipoproteinemia. The unusual turnover characteristics of HDL apoprotein AIV are reviewed, suggesting a dual role for this protein in both triglyceride and cholesterol transport. The striking relationship between very low-density lipoprotein (VLDL) and HDL metabolism is expressed in an inverse association between their respective removal rates.(ABSTRACT TRUNCATED AT 250 WORDS)