Phospholipid transfer protein enhances removal of cellular cholesterol and phospholipids by high-density lipoprotein apolipoproteins

Phospholipid transfer protein activity in two cholestatic patients

CONTEXT

Plasma phospholipid transfer protein mediates the transfer of phospholipids from triglyceride-rich lipoproteins, very low density lipoproteins and low density lipoproteins to high density lipoproteins, a process that is also efficient between high density lipoprotein particles. It promotes a net movement of phospholipids, thereby generating small lipid-poor apolipoprotein AI that contains particles and subfractions that are good acceptors for cell cholesterol efflux.

CASE REPORT

We measured the activity of plasma phospholipid transfer protein in two cholestatic patients, assuming that changes in activity would occur in serum that was positive for lipoprotein X. Both patients presented severe hypercholesterolemia, high levels of low density lipoprotein cholesterol and, in one case, low levels of high density lipoprotein cholesterol and high levels of phospholipid serum. The phospholipid transfer activity was close to the lower limit of the reference interval. To our knowledge, this is the first time such results have been presented. We propose that phospholipid transfer protein activity becomes reduced under cholestasis conditions because of changes in the chemical composition of high density lipoproteins, such as an increase in phospholipids content. Also, lipoprotein X, which is rich in phospholipids, could compete with high density lipoproteins as a substrate for phospholipid transfer protein.

Emerging roles for phospholipid transfer protein in lipid and lipoprotein metabolism

PURPOSE OF REVIEW

This review highlights the recent key advances in our understanding of the role of phospholipid transfer protein in lipid and lipoprotein metabolism.

RECENT FINDINGS

The overexpression of human phospholipid transfer protein in mice is associated with an increase in atherosclerosis. This is consistent with earlier studies using mouse models suggesting that phospholipid transfer protein was pro-atherogenic. The presence of phospholipid transfer protein in macrophages and atherosclerotic lesions suggests that it could be either anti-atherogenic by facilitating lipid efflux or pro-atherogenic by facilitating lipid retention. Phospholipid transfer protein may also be a key player in reverse cholesterol transport, as it interacts with the adenosine triphosphate-binding cassette transporter A1 and facilitates lipid efflux from peripheral cells. Both the release of chymase, a neutral protease, from mast cells and the oxidation of HDL by hypochlorous acid can impair the function of phospholipid transfer protein in reverse cholesterol transport. Studies of phospholipid transfer protein-mediated phospholipid transfer activity in humans support a role for phospholipid transfer protein in hypertriglyceridemia, obesity, diabetes, inflammation and coronary artery disease, and in the modulation of LDL particle density and size. Furthermore, recent evidence suggests that phospholipid transfer protein may play a role in reproductive processes, in lipid and lipoprotein metabolism in the central nervous system, and in neurodegenerative disease.

SUMMARY

Phospholipid transfer protein is emerging as a multifaceted and multifunctional player in lipid and lipoprotein metabolism, but much additional work will be required to understand the significance of these recent findings for clinical practice.

Effects of medium-chain fatty acids and oleic acid on blood lipids, lipoproteins, glucose, insulin, and lipid transfer protein activities

BACKGROUND

Dietary medium-chain fatty acids (MCFAs) are of nutritional interest because they are more easily absorbed from dietary medium-chain triacylglycerols (MCTs) than are long-chain fatty acids from, for example, vegetable oils. It has generally been claimed that MCFAs do not increase plasma cholesterol, although this claim is poorly documented.

OBJECTIVE

We compared the effects of a diet rich in either MCFAs or oleic acid on fasting blood lipids, lipoproteins, glucose, insulin, and lipid transfer protein activities in healthy men.

DESIGN

In a study with a double-blind, randomized, crossover design, 17 healthy young men replaced part of their habitual dietary fat intake with 70 g MCTs (66% 8:0 and 34% 10:0) or high-oleic sunflower oil (89.4% 18:1). Each intervention period lasted 21 d, and the 2 periods were separated by a washout period of 2 wk. Blood samples were taken before and after the intervention periods.

RESULTS

Compared with the intake of high-oleic sunflower oil, MCT intake resulted in 11% higher plasma total cholesterol (P = 0.0005), 12% higher LDL cholesterol (P = 0.0001), 32% higher VLDL cholesterol (P = 0.080), a 12% higher ratio of LDL to HDL cholesterol (P = 0.002), 22% higher plasma total triacylglycerol (P = 0.0361), and higher plasma glucose (P = 0.033). Plasma HDL-cholesterol and insulin concentrations and activities of cholesterol ester transfer protein and phospholipid transfer protein did not differ significantly between the diets.

CONCLUSIONS

Compared with fat high in oleic acid, MCT fat unfavorably affected lipid profiles in healthy young men by increasing plasma LDL cholesterol and triacylglycerol. No changes in the activities of phospholipid transfer protein and cholesterol ester transfer protein were evident.

Elevation of plasma phospholipid transfer protein increases the risk of atherosclerosis despite lower apolipoprotein B-containing lipoproteins

Plasma phospholipid transfer protein (PLTP) transfers phospholipids between lipoproteins and mediates HDL conversion. PLTP-overexpressing mice have increased atherosclerosis. However, mice do not express cholesteryl ester transfer protein (CETP), which is involved in the same metabolic pathways as PLTP. Therefore, we studied atherosclerosis in heterozygous LDL receptor-deficient (LDLR(+/-)) mice expressing both human CETP and human PLTP. We used two transgenic lines with moderately and highly elevated plasma PLTP activity. In LDLR(+/-)/huCETPtg mice, cholesterol is present in both LDL and HDL. Both are decreased in LDLR(+/-)/huCETPtg/huPLTPtg mice (>50%). An atherogenic diet resulted in high levels of VLDL+LDL cholesterol. PLTP expression caused a strong PLTP dose-dependent decrease in VLDL and LDL cholesterol (-26% and -69%) and a decrease in HDL cholesterol (-70%). Surprisingly, atherosclerosis was increased in the two transgenic lines with moderately and highly elevated plasma PLTP activity (1.9-fold and 4.4-fold, respectively), indicating that the adverse effect of the reduction in plasma HDL outweighs the beneficial effect of the reduction in apolipoprotein B (apoB)-containing lipoproteins. The activities of the antiatherogenic enzymes paraoxonase and platelet-activating factor acetyl hydrolase were both PLTP dose-dependently reduced ( approximately -33% and -65%, respectively). We conclude that expression of PLTP in this animal model results in increased atherosclerosis in spite of reduced apoB-containing lipoproteins, by reduction of HDL and of HDL-associated antioxidant enzyme activities.

Quantitative Trait Locus Mapping of Genes That Regulate Phospholipid Transfer Activity in SM/J and NZB/BlNJ Inbred Mice

OBJECTIVE

Phospholipid transfer protein (PLTP), an important protein in the transfer of phospholipids between lipoprotein particles and in the remodeling of HDL, is regulated at both the transcriptional and the protein level. We performed quantitative trait locus (QTL) analysis to identify genomic loci regulating PLTP activity in mice.

METHODS AND RESULTS

Plasma PLTP activity was measured in 217 male F2 progeny from a SM/J x NZB/B1NJ intercross. Two QTL for plasma PLTP activity in mice fed chow (Pltpq1 and Pltpq2) were found on chromosomes 3 (34 cM, logarithm of odds [LOD] 3.5) and 10 (66 cM, LOD 4.1); two additional QTL in mice fed atherogenic diet (Pltpq3 and Pltpq4) were found on chromosomes 9 (56 cM, LOD 4.5) and 15 (34 cM, LOD 5.0); and one QTL (Pltiq1) for the inducibility of PLTP activity was found on chromosome 4 (70 cM, LOD 3.7). Several candidate genes for these 5 QTL were tested by sequence comparison and expression studies.

CONCLUSIONS

We identified five significant loci involved in PLTP activity in the mouse and provided supporting evidence for the candidacy of Nr1h4 and Apof as the genes underlying Pltpq2.

Periodontitis decreases the antiatherogenic potency of high density lipoprotein

Periodontitis, a consequence of persistent bacterial infection and chronic inflammation, has been suggested to predict coronary heart disease (CHD). The aim of this study was to investigate the impact of periodontitis on HDL structure and antiatherogenic function in cholesterol efflux in vitro. HDL was isolated from 30 patients (age 43.6 +/- 6.1 years, mean +/- SD) with periodontitis before and after (3.2 +/- 1.4 months) periodontal treatment. The capacity of HDL for cholesterol efflux from macrophages (RAW 264.7), HDL composition, and key proteins of HDL metabolism were determined. After periodontal treatment, phospholipid transfer protein (PLTP) activity was 6.2% (P<0.05) lower, and serum HDL cholesterol concentration, PLTP mass, and cholesteryl ester transfer protein activity were 10.7% (P<0.001), 7.1% (P=0.078), and 19.4% (P<0.001) higher, respectively. The mean HDL2/HDL3 ratio increased from 2.16 +/- 0.87 to 3.56 +/- 0.48 (P<0.05). HDL total phospholipid mass and sphingomyelin-phosphatidylcholine ratio were 7.4% (P<0.05) and 36.8% (P<0.001) higher, respectively. The HDL-mediated cholesterol efflux tended to be higher after periodontal treatment; interestingly, this increase was significant (P<0.05) among patients whose C-reactive protein decreased (53.7% reduction, P=0.015) and who were positive by PCR for Actinobacillus actinomycetemcomitans. These results suggest that periodontitis causes similar, but milder, changes in HDL metabolism than those that occur during the acute-phase response and that periodontitis may diminish the antiatherogenic potency of HDL, thus increasing the risk for CHD.

Phospholipid transfer protein interacts with and stabilizes ATP-binding cassette transporter A1 and enhances cholesterol efflux from cells

Phospholipid lipid transfer protein (PLTP) is ubiquitously expressed in animal tissues and plays multiple roles in lipoprotein metabolism, but the function of peripheral PLTP is still poorly understood. Here we show that one of its possible functions is to transport cholesterol and phospholipids from cells to lipoprotein particles by a process involving PLTP interactions with cellular ATP-binding cassette transporter A1 (ABCA1). When ABCA1 was induced in murine macrophages or ABCA1-transfected baby hamster kidney cells, PLTP gained the ability to promote cholesterol and phospholipid efflux from cells. Although PLTP alone had lipid efflux activity, its maximum activity was observed in the presence of high density lipoprotein particles. Pulsechase studies showed that the interaction of PLTP with ABCA1-expressing cells played a role in promoting lipid efflux. Overexpression of ABCA1 dramatically increased binding of both PLTP and apoA-I to common sites on the cell surface. Both PLTP and apoA-I were covalently cross-linked to ABCA1, each protein blocked cross-linking of the other, and both PLTP and apoA-I stabilized ABCA1 protein. These results are consistent with PLTP and apoA-I binding to ABCA1 at the same or closely related sites. Thus, PLTP mimics apolipoproteins in removing cellular lipids by the ABCA1 pathway, except that PLTP acts more as an intermediary in the transfer of cellular lipids to lipoprotein particles.

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.

Decreased PLTP mass but elevated PLTP activity linked to insulin resistance in HTG: effects of bezafibrate therapy

Hypertriglyceridemia (HTG) is associated with insulin resistance, increased cholesteryl ester transfer (CET), and low HDL cholesterol. Phospholipid transfer protein (PLTP) may be involved in these relationships. Associations between CET, lipids, insulin resistance, CETP and PLTP activities, and PLTP mass were investigated in 18 HTG patients and 20 controls. Effects of 6 weeks of bezafibrate treatment were studied in HTG patients. HTG patients had higher serum triglycerides, insulin resistance, free fatty acid (FFA), and CET, lower levels of HDL cholesterol (-44%) and PLTP mass (-54%), and higher CETP (+20%) and PLTP activity (+48%) than controls. Bezafibrate reduced triglycerides, CET (-37%), insulin resistance (-53%), FFA (-48%), CETP activity (-12%), PLTP activity (-8%), and increased HDL cholesterol (+27%), whereas PLTP mass remained unchanged. Regression analysis showed a positive contribution of PLTP mass (P = 0.001) but not of PLTP activity to HDL cholesterol, whereas insulin resistance positively contributed to PLTP activity (P < 0.01). Bezafibrate-induced change in CET and HDL cholesterol correlated with changes in CETP activity and FFAs, but not with change in PLTP activity. Bezafibrate-induced change in PLTP activity correlated with change in FFAs (r = 0.455, P = 0.058). We propose that elevated PLTP activity in HTG is related to insulin resistance and not to increased PLTP mass. Bezafibrate-induced diminished insulin resistance is associated with a reduction of CET and PLTP activity.

Lipoprotein lipase and hepatic lipase: their relationship with HDL subspecies Lp(A-I) and Lp(A-I,A-II)

HDL subspecies Lp(A-I) and Lp(A-I,A-II) have different anti-atherogenic potentials. To determine the role of lipoprotein lipase (LPL) and hepatic lipase (HL) in regulating these particles, we measured these enzyme activities in 28 healthy subjects with well-controlled Type 1 diabetes, and studied their relationship with Lp(A-I) and Lp(A-I,A-II). LPL was positively correlated with the apolipoprotein A-I (apoA-I), cholesterol, and phospholipid mass in total Lp(A-I), and with the apoA-I in large Lp(A-I) (r >or= 0.58, P >or= 0.001). HL was negatively correlated with all the above Lp(A-I) parameters plus Lp(A-I) triglyceride (r >or= -0.53, P <or= 0.003). No correlation was detected between LPL and Lp(A-I,A-II). However, HL was inversely correlated with total Lp(A-I,A-II) phospholipid, and with large Lp(A-I,A-II) (r >or= 0.50, P <or= 0.006). Similar studies were performed with phospholipid transfer protein (PLTP). Only total Lp(A-I) triglyceride in women (not men) (r = 0.71, P = 0.009) was significantly correlated with PLTP activity. These observations indicate that LPL and HL play major roles in determining the level and composition of plasma Lp(A-I), particularly large Lp(A-I), but not with Lp(A-I,A-II) level. Furthermore, select correlations of LPL and/or HL with the apoA-I, cholesterol, and triglyceride of Lp(A-I) but not Lp(A-I,A-II) imply that the apoA-I and lipid of Lp(A-I) and Lp(A-I,A-II) are not fully equilibrated.

Phospholipid transfer protein is present in human atherosclerotic lesions and is expressed by macrophages and foam cells

Phospholipid transfer protein (PLTP) in plasma promotes phospholipid transfer from triglyceride-rich lipoproteins to HDL and plays a major role in HDL remodeling. Recent in vivo observations also support a key role for PLTP in cholesterol metabolism. Our immunohistochemical analysis of human carotid endarterectomy samples identified immunoreactive PLTP in areas that colocalized with CD68-positive macrophages, suggesting that PLTP could be produced locally by intimal macrophages. Using RT-PCR, Western blot analysis with a monoclonal anti-PLTP antibody, and a PLTP activity assay, we observed PLTP mRNA and protein expression in human macrophages. In adherent peripheral blood human macrophages, this PLTP expression was increased by culture with granulocyte macrophage colony-stimulating factor. Incubation of macrophages with acetylated-LDL induced an increase in PLTP mRNA and protein expression that paralleled cholesterol loading. PLTP expression was observed in elicited mouse peritoneal macrophages and in cultured Raw264.7 cells as well. Thus, this study demonstrates that PLTP is expressed by macrophages, is regulated by cholesterol loading, and is present in atherosclerotic lesions.

Cell-associated and extracellular phospholipid transfer protein in human coronary atherosclerosis

BACKGROUND

Phospholipid transfer protein (PLTP) plays an important role in HDL particle metabolism and may modulate hepatic secretion of apolipoprotein B-containing lipoproteins. However, whether PLTP might participate directly in human atherosclerotic lesion formation is unknown.

METHODS AND RESULTS

The cellular and extracellular distributions of PLTP were determined in normal and atherosclerotic human coronary lesions with a monoclonal antibody to human PLTP. Cell types (smooth muscle cells [SMCs] or macrophages), apolipoproteins (apoA-I, apoB, and apoE), and extracellular matrix proteoglycans (biglycan and versican) were identified on adjacent sections with monospecific antibodies. Minimal extracellular PLTP was detected in nonatherosclerotic coronary arteries, but extracellular and cellular PLTP immunostaining was widespread in atherosclerotic lesions. PLTP was detected in foam cell SMCs and in foam cell macrophages, which suggests that cellular cholesterol accumulation might increase PLTP expression in both cell types. This was confirmed by in vitro studies demonstrating that cholesterol loading of macrophages leads to 2- to 3-fold increases in PLTP steady-state mRNA levels, protein expression, and activity. PLTP also was detected in an extracellular distribution, colocalizing with apoA-I, apoB, apoE, and the vascular proteoglycan biglycan. In gel mobility shift assays, both active and inactive recombinant PLTP markedly increased HDL binding to biglycan, which suggests that PLTP may mediate lipoprotein binding to proteoglycans independent of its phospholipid transfer activity.

CONCLUSIONS

PLTP is present in human atherosclerotic lesions, and its distribution suggests roles for PLTP in both cellular cholesterol metabolism and lipoprotein retention on extracellular matrix.

High-cholesterol diets induce changes in lipid composition of rat erythrocyte membrane including decrease in cholesterol, increase in alpha-tocopherol and changes in fatty acids of phospholipids

Effects of high dietary cholesterol on erythrocyte membrane lipids were studied. Feeding rats with a diet containing 0.5% cholesterol and 0.15% sodium cholate for two weeks induced changes in erythrocyte membrane lipids including a decrease in cholesterol, an increase in alpha-tocopherol (alpha-Toc) and changes in the fatty acid composition of phospholipids. Oleic acid and linoleic acid increased, while arachidonic acid decreased in phosphatidylcholine. Saturated fatty acids decreased and unsaturated fatty acids increased in phosphatidylethanolamine. Almost the same changes in membrane lipids were also noted after six weeks of feeding rats with the diet. A diet containing 0.5% cholesterol but without sodium cholate caused a decrease in erythrocyte cholesterol and an increase in erythrocyte alpha-Toc after two weeks of feeding, as compared to the basal diet, indicating that high dietary cholesterol, but not sodium cholate, was responsible for these changes in the erythrocyte membrane.

Widespread distribution of PLTP in human CNS: evidence for PLTP synthesis by glia and neurons, and increased levels in Alzheimer's disease

Plasma phospholipid transfer protein (PLTP) is one of the key proteins in lipid and lipoprotein metabolism. We examined PLTP distribution in human brain using PLTP mRNA dot-blot, Northern blot, immunohistochemistry (IHC), Western blot, and phospholipid transfer activity assay analyses. PLTP mRNA of 1.8 kb was widely distributed in all the examined regions of the central nervous system at either comparable or slightly lower levels than in the other major organs, depending on the region. Cerebrospinal fluid phospholipid transfer activity represented 15% of the plasma activity, indicating active PLTP synthesis in the brain. Western blot and phosholipid transfer activity assay demonstrated secretion of active PLTP by neurons, microglia, and astrocytes in culture. IHC demonstrated PLTP presence in neurons, astrocytes, microglia, and oligodendroglia. Some neuronal groups, such as nucleus hypoglossus and CA2 neurons in hippocampus, ependymal layer, and choroid plexus were particularly strongly stained, with substantial glial and neuropil immunostaining throughout the brain. Comparison between brain tissues from patients with Alzheimer's disease (AD) and nonAD subjects revealed a significant increase (P = 0.02) in PLTP levels in brain tissue homogenates and increased PLTP immunostaining in AD.

Degradation of phospholipid transfer protein (PLTP) and PLTP-generated pre-beta-high density lipoprotein by mast cell chymase impairs high affinity efflux of cholesterol from macrophage foam cells

Human atherosclerotic lesions contain mast cells filled with the neutral protease chymase. Here we studied the effect of human chymase on (i) phospholipid transfer protein (PLTP)-mediated phospholipid (PL) transfer activity, and (ii) the ability of PLTP to generate pre-beta-high density lipoprotein (HDL). Immunoblot analysis of PLTP after incubation with chymase for 6 h revealed, in addition to the original 80-kDa band, four specific proteolytic fragments of PLTP with approximate molecular masses of 70, 52, 48, and 31 kDa. This specific pattern of PLTP degradation remained stable for at least 24 h of incubation with chymase. Such proteolyzed PLTP had reduced ability (i) to transfer PL from liposome donor particles to acceptor HDL(3) particles, and (ii) to facilitate the formation of pre-beta-HDL. However, when PLTP was incubated with chymase in the presence of HDL(3), only one major cleavage product of PLTP (48 kDa) was generated, and PL transfer activity was almost fully preserved. Moreover, chymase effectively depleted the pre-beta-HDL particles generated from HDL(3) by PLTP and significantly inhibited the high affinity component of cholesterol efflux from macrophage foam cells. These results suggest that the mast cells in human atherosclerotic lesions, by secreting chymase, may prevent PLTP-dependent formation of pre-beta-HDL particles from HDL(3) and so impair the anti-atherogenic function of PLTP.

Postprandial variations in the cholesteryl ester transfer protein activity, phospholipid transfer protein activity and plasma cholesterol efflux capacity in normolipidemic men

BACKGROUND AND AIM

Plasma cholesterol efflux capacity is stimulated during postprandial (PP) hypertriglycerdemia. Plasma cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) are the key proteins in lipoprotein metabolism and remodelling, but their role during the PP cholesterol efflux process remains indeterminate. The aim of this study was to determine the effect of a fatty meal intake on plasma CETP and PLTP activities, and the capacity of plasma to promote cholesterol efflux, as well as to evaluate the relationship between these three key mechanisms of the reverse cholesterol transport process.

METHODS AND RESULTS

CETP and PLTP activities and the cholesterol efflux capacity of plasma were measured over eight hours following a fatty meal (1000 kcal, 62% fat) in 13 normolipidemic men. CETP activity and the cholesterol efflux capacity of plasma from Fu5AH cells increased after the meal, reaching a maximum after eight hours (respectively 32%, p = 0.06, and 6.5%, p = 0.045), whereas PLTP activity remained unchanged. CETP and PLTP activities did not correlate with plasma cholesterol efflux capacity in the fasting or PP state. Plasma CETP activity in the fasting state positively correlated with the plasma non-esterified fatty acid (NEFA) levels, but no correlation was found with any lipid or apolipoprotein postprandially. The cholesterol efflux capacity of plasma correlated positively with high-density lipoprotein (HDL) components, the best correlation being with the HDL phospholipid fraction in both the fasting and PP states.

CONCLUSIONS

These findings suggest that plasma CETP and PLTP activities in healthy normolipidemic subjects are differently regulated in the PP state, and are not correlated with the increased cholesterol efflux capacity of PP plasma. HDL-phospholipid remains the key factor in the regulation of the capacity of plasma to promote Fu5AH cell cholesterol efflux.

Association of plasma phospholipid transfer protein activity with IDL and buoyant LDL: impact of gender and adiposity

Current data suggest that phospholipid transfer protein (PLTP) has multiple metabolic functions, however, its physiological significance in humans remains to be clarified. To provide further insight into the role of PLTP in lipoprotein metabolism, plasma PLTP activity was measured, and lipoproteins were analyzed in 134 non-diabetic individuals on a controlled diet. Insulin sensitivity index (Si) and body fat composition were also determined. Plasma PLTP activity was comparable between men (n=56) and women (n=78). However, in women but not in men, plasma PLTP activity was positively correlated with cholesterol, triglyceride, low density lipoprotein (LDL) cholesterol, and apolipoprotein (apo) B (r=0.38-0.45, P< or =0.001), and with body mass index (BMI), subcutaneous and intra-abdominal fat (SCF, IAF) (r=0.27-0.29, P<0.02). Among the different apo B-containing lipoproteins (LpB) in women, PLTP was most highly correlated with intermediate density lipoproteins (IDL) and buoyant LDL (r=0.45-0.46, P<0.001). The correlation with IDL was significant only in women with BMI < or =27.5 kg/m(2) (n=56). In men with BMI < or =27.5 kg/m(2) (n=35), PLTP activity was significantly correlated with buoyant LDL (r=0.40, P<0.02) and high density lipoprotein (HDL) (r=0.43, P<0.01). These data provide evidence for a role of PLTP in LpB metabolism, particularly IDL and buoyant LDL. They also suggest that gender and obesity-related factors can modulate the impact of PLTP on LpB.

Functional analysis of the transcriptional activity of the mouse phospholipid transfer protein gene

Phospholipid transfer protein (PLTP) plays an important role in the metabolism of plasma high density lipoprotein. The mouse gene encoding PLTP and its promoter region has been cloned in our laboratory. The present study was conducted to functionally analyze the transcriptional regulation of the mouse PLTP gene. The results indicated that DNA sequences between -245 and -69 were responsible for the full promoter activity and binding motifs for transcription factor Sp1 and AP-2 within this functional promoter region were synergistically essential for the basal transcription. The transcriptional activity of this gene was significantly increased by chenodeoxycholic acid and fenofibrate, suggesting that transcription factor farnesoid X-activated receptor (FXR) and peroxisome proliferator-activated receptor (PPAR) are likely involved in the transcriptional regulation. DNA sequence analysis suggests that DNA sequences from -407 to -395 and from -393 to -381 are homologous to the recognition motifs of FXR, and those from -859 to -847 and from -309 to -297 are similar to the potential binding motif for PPAR. These findings provide a molecular basis for further investigation of the physiological function and regulation of the PLTP gene in mice.

Plasma phospholipid transfer protein activity in patients with low HDL and cardiovascular disease treated with simvastatin and niacin

Plasma phospholipid transfer protein (PLTP) is an important modulator of high-density lipoprotein (HDL) metabolism, regulating its particle size, composition, and mass. In patients with low HDL and cardiovascular disease (CVD), plasma PLTP activity is positively correlated with the concentration of HDL particles containing apo A-I but not apo A-II (Lp(A-1)). We recently completed a study to determine the effect of simvastatin and niacin (S-N) therapy on disease progression/regression in these patients, and found that this therapy selectively increased Lp(A-I). To determine if PLTP was also increased with this drug therapy, we measured the PLTP activity in the plasma of 30 of these patients obtained at baseline and after 12 months of therapy, and compared the changes to a similar group of 31 patients who received placebo for the drugs. No significant increase in PLTP activity was observed in either group of patients. However, changes in apo A-I and A-II between these two time points were correlated with the corresponding change in PLTP activity. The correlation coefficients were r=0.57 (P=0.001) and r=0.43 (P=0.02) for apo A-I, and r=0.54 (P=0.002) and r=0.41 (P=0.02) for apo A-II in the placebo and S-N group, respectively. At baseline, PLTP activity correlated positively with the percent of plasma apo A-I associated with Lp(A-I) (r=0.38, P=0.04) and the amounts of apo A-I in these particles (r=0.43, P=0.02). These relationships persisted in patients who took placebo for 12 months (r=0.46, P=0.009 and r=0.37, P=0.04, respectively), but was attenuated in those treated with S-N. These data indicate that S-N-induced increase in Lp(A-I) was PLTP-independent. It also confirms our previous observation that an interrelationship exists between PLTP and apo-specific HDL particle subclasses in CVD patients with low HDL, and that this relationship is altered by drug intervention.

The role of plasma phospholipid transfer protein (PLTP) in HDL remodeling in acute-phase patients

During reverse cholesterol transport plasma phospholipid transfer protein (PLTP) converts high density lipoprotein(3) (HDL(3)) into two new subpopulations, HDL(2)-like particles and pre-beta-HDL. The acute-phase response is accompanied with dramatic changes in lipid metabolism including alterations in HDL concentration, composition, and thereby its function as a substrate for HDL remodeling proteins in circulation. To evaluate how acute-phase HDL (AP-HDL) functions in PLTP-mediated HDL conversion, we collected plasma samples from patients with severe acute-phase response (n=17), and from healthy controls (n=30). Subsequently, total HDL (1.063<d<1.21 g/ml) was isolated from patients and controls and incubated in the absence and presence of purified PLTP. The results show that HDL isolated from the acute-phase patients is converted by PLTP in vitro in a corresponding manner as normal HDL. In the combined population, C-reactive protein correlated significantly with lecithin-cholesterol acyltransferase (LCAT) activity (r=-0.53), cholesterol ester transfer protein activity (r=-0.80), PLTP activity (r=0.44), and PLTP mass (r=-0.66). When compared to the controls, the patients had 31% higher PLTP activity, but 52% lower PLTP mass leading to a 165% higher PLTP specific activity in the patients. The present data indicate that during the acute-phase response, plasma PLTP activity and mass are strongly affected by the lipoprotein distribution as well as lipid composition. We suggest that the decrease of HDL during the acute phase is caused by reduced LCAT and increased PLTP activities both increasing the plasma levels of lipid-poor apoA-I particles.

Intracellular cholesterol and phospholipid trafficking: comparable mechanisms in macrophages and neuronal cells

During the past ten years considerable evidences have accumulated that in addition to monocytes/macrophages, that are implicated in innate immunity and atherogenesis, neuronal cells also exhibit an extensive cellular metabolism. The present study focuses on the major protein players that establish cellular distribution of cholesterol and phospholipids. Evidences are provided that neuronal cells and monocytes/macrophages are equipped with comparable intracellular lipid trafficking mechanisms. Selected examples are presented that trafficking dysfunctions lead to disease development, such as Tangier disease and Niemann-Pick disease type C, or contribute to the pathogenesis of diseases such as Alzheimer disease and atherosclerosis.

Glucose regulates the transcription of human genes relevant to HDL metabolism: responsive elements for peroxisome proliferator-activated receptor are involved in the regulation of phospholipid transfer protein

Phospholipid transfer protein (PLTP) plays an important role in human plasma HDL metabolism. Clinical data have recently indicated that plasma PLTP activity and mass were both higher in diabetic patients concomitant with hyperglycemia. The present study shows that high glucose increases both PLTP mRNA and functional activity in HepG2 cells, due to a significant increase in the promoter activity of human PLTP gene. The glucose-responsive elements are located between -759 and -230 of the PLTP 5'-flanking region, within which two binding motifs (-537 to -524 and -339 to -327) for either peroxisome proliferator-activated receptor or farnesoid X-activated receptor are involved in this glucose-mediated transcriptional regulation. This finding suggests that high glucose upregulates the transcription of human PLTP gene via nuclear hormone receptors. In addition, high glucose increases mRNA levels for several genes that are functionally important in HDL metabolism, including human ATP-binding cassette transporter A1, apolipoprotein A-I, scavenger receptor BI, and hepatic lipase. The functional promoter activities of these genes are enhanced by high glucose in three cell lines tested, indicating that glucose may also regulate these genes at the transcriptional level. Our findings provide a molecular basis for a role of hyperglycemia in altered HDL metabolism.

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)

Influence of caveolin-1 on cellular cholesterol efflux mediated by high-density lipoproteins

Caveolin-1 is a principal structural component of caveolae membranes. These membrane microdomains participate in the regulation of signaling, transcytosis, and cholesterol homeostasis at the plasma membrane. In the present study, we determined the effect of caveolin-1 expression on cellular cholesterol efflux mediated by high-density lipoprotein (HDL). We evaluated this effect in parental NIH/3T3 cells as well as in two transformed NIH/3T3 cell lines in which caveolin-1 protein levels are dramatically downregulated. Compared with parental NIH/3T3 cells, these two transformed cell lines effluxed cholesterol more rapidly to HDL. In addition, NIH/3T3 cells harboring caveolin-1 antisense also effluxed cholesterol more rapidly to HDL. However, this effect was not due to changes in total cellular cholesterol content. We further showed that chronic HDL exposure reduced caveolin-1 protein expression in NIH/3T3 cells. HDL exposure also inhibited caveolin-1 promoter activity, suggesting a direct negative effect of HDL on caveolin-1 gene transcription. Moreover, we showed that HDL-induced downregulation of caveolin-1 prevents the uptake of oxidized low-density lipoprotein in human endothelial cells. These data suggest a novel proatherogenic role for caveolin-1, i.e., regarding the uptake and/or transcytosis of modified lipoproteins.

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.

Effects of high-density lipoprotein(2) on cholesterol transport and acyl-coenzyme A:cholesterol acyltransferase activity in P388D1 macrophages

High-density lipoproteins are the putative vehicles for cholesterol removal from monocyte-derived macrophages, which are an important cell type in all stages of atherosclerosis. The role of HDL(2), an HDL subclass that accounts for most variation in plasma HDL-cholesterol concentration, in cholesterol metabolism in monocyte-derived macrophages is not known. In this study, the dose-dependent effects of HDL(2) on cellular cholesterol mass, efflux, and esterification, and on cellular cholesteryl ester (CE) hydrolysis using the mouse macrophage P388D1 cell line was investigated. HDL(2) at low concentrations (40 microg protein/ml) decreased CE content without affecting cellular free cholesterol content (FC), CE hydrolysis, or cholesterol biosynthesis. In addition, HDL(2) at low concentrations reduced cellular acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity and increased FC efflux from macrophages. Thus, HDL(2) has two potential roles in reverse cholesterol transport. In one, HDL(2) is an acceptor of macrophage FC. In the other, more novel role, HDL(2) increases the availability of macrophage FC through the inhibition of ACAT. Elucidation of the mechanism by which HDL(2) inhibits ACAT could identify new therapeutic targets that enhance the transfer of cholesterol from macrophages to the liver.

Release of cellular cholesterol: molecular mechanism for cholesterol homeostasis in cells and in the body

Most mammalian somatic cells are unable to catabolize cholesterol and therefore need to export it in order to maintain sterol homeostasis. This mechanism may also function to reduce excessively accumulated cholesterol, which would thereby contribute to prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) has been believed to play a main role in this reaction based on epidemiological evidence and in vitro experimental data. At least two independent mechanisms are identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from cell surface. Cholesterol molecules desorbed from cells can be trapped by various extracellular acceptors including various lipoproteins and albumin, and extracellular cholesterol esterification mainly on HDL may provide a driving force for the net removal of cell cholesterol by maintaining a cholesterol gradient between lipoprotein surface and cell membrane. The other is apolipoprotein-mediated process to generate new HDL by removing cellular phospholipid and cholesterol. The reaction is initiated by the interaction of lipid-free or lipid-poor helical apolipoproteins with cellular surface resulting in assembly of HDL particles with cellular phospholipid and incorporation of cellular cholesterol into the HDL being formed. Thus, HDL has dual functions as an active cholesterol acceptor in the diffusion-mediated pathway and as an apolipoprotein carrier for the HDL assembly reaction. The impairment of the apolipoprotein-mediated reaction was found in Tangier disease and other familial HDL deficiencies to strongly suggest that this is a main mechanism to produce plasma HDL. The causative mutations for this defect was identified in ATP binding cassette transporter protein A1, as a significant step for further understanding of the reaction and cholesterol homeostasis.

Acceleration of reverse cholesterol transport

A low level of high-density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease. Levels of HDL cholesterol and composition of HDL subclasses in plasma are regulated by many factors, including apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. Reverse transport of cholesterol from cells of the arterial wall to the liver is an important mechanism by which HDL exerts its anti-atherogenic properties. Enhancement of reverse cholesterol transport is considered as a potential target for anti-atherosclerotic drug therapy. It is suggested, however, that the serum level of HDL cholesterol does not necessarily reflect the efficacy of reverse cholesterol transport.

Reverse cholesterol transport in diabetes mellitus

There are epidemiological data and experimental animal models relating the development of premature atherosclerosis with defects of the reverse cholesterol transport (RCT) system. In this regard, the plasma concentrations of the high density lipoprotein (HDL) subfractions, of cholesteryl ester transfer protein (CETP), as well as the activity of the enzyme lecithin-cholesterol acyl transferase (LCAT) play critical roles. However, there has been plenty of evidence that atherosclerosis in diabetes mellitus (DM) is ascribed to a greater arterial wall cell uptake of modified apoB-containing lipoproteins whereas a primary or predominant defect of the RCT system is still a subject of debate. In other words, in spite of the fact that in DM the composition and rates of metabolism of the HDL particles are greatly altered and display a diminished in vitro efficiency to remove cell cholesterol, definitive in vivo demonstration of the importance of this fact in atherogenesis is lacking. Furthermore, the roles played by LCAT and CETP in RCT in DM are difficult to interpret because the in vitro procedures of measurement utilized have either been inadequate, or inappropriately interpreted. Knock-out or transgenic mice are much needed models to investigate the roles of LCAT, CETP, phospholipid transfer protein (PLTP), and of a CETP inhibitor in the development of atherosclerosis of experimental DM.

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.

Cholesterol efflux regulatory protein, Tangier disease and familial high-density lipoprotein deficiency

Cellular cholesterol efflux, by which cholesterol is transported from peripheral cells to HDL acceptor molecules for transport to the liver, is the first step of reverse cholesterol transport. Two genetic disorders, Tangier disease and some cases of familial HDL deficiency, have defects of cellular cholesterol efflux. The recent discovery of mutations in the ABC1 gene, which encodes the cholesterol efflux regulatory protein, in both these disorders establishes cholesterol efflux regulatory protein as a rate-limiting factor in reverse cholesterol transport.

Genes influencing HDL metabolism: new perspectives and implications for atherosclerosis prevention

Atherosclerotic cardiovascular disease (ASCVD) is the most common cause of morbidity and mortality in Western societies. Current therapies, such as reduction of plasma cholesterol, significantly reduce, but do not come close to eliminating, the complications of ASCVD. Therefore, novel therapeutic approaches to the prevention of acute coronary events and progression of atherosclerosis are still needed. The complex metabolism of high density lipoproteins represents an attractive potential target for therapeutic intervention. Here, we will discuss those components of the high density lipoprotein metabolism and lipid transport pathways that are potential preventative or therapeutic targets for ASCVD.

DNA sequences responsible for reduced promoter activity of human phospholipid transfer protein by fibrate

Phospholipid transfer protein (PLTP) plays an important role in plasma lipid and lipoprotein metabolism. We have previously cloned and characterized the promoter region of the human PLTP gene. The present study was conducted to determine if the promoter activity of the human PLTP gene is affected by fibrate, a hypolipidemic drug, and to identify DNA sequences that are responsible for the effect. The results indicated that the promoter activity of the PLTP gene was significantly reduced by fenofibrate, and the area that was mainly responsive to the reducing effect by fibrate was located between -377 and -230 of the 5'-flanking region. The DNA sequence analysis suggested that each area of the DNA sequences from -342 to -323 and from -322 to -299 has two repeated sequences, which are inverted and homologous to the recognition motif of peroxisome proliferator-activated receptor (PPAR), namely the PPAR-responsive element (PPRE). Mutagenesis of these PPRE-like sequences, especially that at -322 to -299, abolished most of the reducing effects of fibrate on the PLTP promoter activity. These findings strongly suggest that the PPRE-like elements are responsible for the reduced promoter activity of the human PLTP gene by fibrate.