Cholesteryl ester transfer protein and phospholipid transfer protein have nonoverlapping functions in vivo

Plasma phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein (CETP) are homologous molecules that mediate neutral lipid and phospholipid exchange between plasma lipoproteins. Biochemical experiments suggest that only CETP can transfer neutral lipids but that there could be overlap in the ability of PLTP and CETP to transfer or exchange phospholipids. Recently developed PLTP gene knock-out (PLTP0) mice have complete deficiency of plasma phospholipid transfer activity and markedly reduced high density lipoprotein (HDL) levels. To see whether CETP can compensate for PLTP deficiency in vivo, we bred the CETP transgene (CETPTg) into the PLTP0 background. Using an in vivo assay to measure the transfer of [(3)H]PC from VLDL into HDL or an in vitro assay that determined [(3)H]PC transfer from vesicles into HDL, we could detect no phospholipid transfer activity in either PLTP0 or CETPTg/PLTP0 mice. On a chow diet, HDL-PL, HDL-CE, and HDL-apolipoprotein AI in CETPTg/PLTP0 mice were significantly lower than in PLTP0 mice (45 +/- 7 versus 79 +/- 9 mg/dl; 9 +/- 2 versus 16 +/- 5 mg/dl; and 51 +/- 6 versus 100 +/- 9, arbitrary units, respectively). Similar results were obtained on a high fat, high cholesterol diet. These results indicate 1) that there is no redundancy in function of PLTP and CETP in vivo and 2) that the combination of the CETP transgene with PLTP deficiency results in an additive lowering of HDL levels, suggesting that the phenotype of a human PLTP deficiency state would include reduced HDL levels.     

Phospholipid transfer protein

A role for phospholipid transfer protein (PLTP) in HDL remodelling and in the formation of pre-beta-HDL is now well established, both in vivo and in vitro. Over-expression of human PLTP in C57BL6 mice lowers plasma HDL levels, probably because of increased HDL catabolism. Despite these low HDL levels, plasma from these mice mitigates cholesterol accumulation in macrophages and has increased potential for pre-beta-HDL formation. Plasma HDL concentration is also decreased in PLTP knockout mice. These intriguing observations can be explained by recent studies that indicate that PLTP is not only involved in remodelling of HDL subfractions but also in VLDL turnover. The role of PLTP in atherogenesis and VLDL synthesis was demonstrated in transgenic mouse models with increased susceptibility for the development of atherosclerosis, bred into PLTP knockout mice. The data clearly show that PLTP can be proatherogenic. As mentioned above, however, PLTP may have antiatherogenic potential in wild-type C57BL6 mice. Information regarding the role and regulation of PLTP in human (patho)physiology is still relatively sparse but accumulating rapidly. PLTP activity is elevated in diabetes mellitus (both type 1 and type 2), obesity and insulin resistance.     

Interfacial and lipid transfer properties of human phospholipid transfer protein: implications for the transfer mechanism of phospholipids

In circulation the phospholipid transfer protein (PLTP) facilitates the transfer of phospholipid-rich surface components from postlipolytic chylomicrons and very low density lipoproteins (VLDL) to HDL and thereby regulates plasma HDL levels. To study the molecular mechanisms involved in PLTP-mediated lipid transfer, we studied the interfacial properties of PLTP using Langmuir phospholipid monolayers and asymmetrical flow field-flow fractionation (AsFlFFF) to follow the transfer of 14C-labeled phospholipids and [35S]PLTP between lipid vesicles and HDL particles. The AsFlFFF method was also used to determine the sizes of spherical and discoidal HDL particles and small unilamellar lipid vesicles. In Langmuir monolayer studies high-activity (HA) and low-activity (LA) forms of PLTP associated with fluid phosphatidylcholine monolayers spread at the air/buffer interphase. Both forms also mediated desorption of [14C]dipalmitoylphosphatidylcholine (DPPC) from the phospholipid monolayer into the buffer phase, even when it contained no physiological acceptor such as HDL. After the addition of HDL3 to the buffer, HA-PLTP caused enhanced lipid transfer to them. The particle diameter of HA-PLTP was approximately 6 nm and that of HDL3 approximately 8 nm as determined by AsFlFFF analysis. Using this method, it could be demonstrated that in the presence of HA-PLTP, but not LA-PLTP, [14C]DPPC was transferred from small unilamellar vesicles (SUV) to acceptor HDL3 molecules. Concomitantly, [35S]-HA-PLTP was transferred from the donor to acceptor, and this transfer was not observed for its low-activity counterpart. These observations suggest that HA-PLTP is capable of transferring lipids by a shuttle mechanism and that formation of a ternary complex between PLTP, acceptor, and donor particles is not necessary for phospholipid transfer.     

Structural and functional determinants of human plasma phospholipid transfer protein activity as revealed by site-directed mutagenesis of charged amino acids

Human plasma phospholipid transfer protein (PLTP) exchanges phospholipids between lipoproteins and remodels high-density lipoproteins (HDLs). We determined phospholipid transfer activity and HDL binding ability in wild-type PLTP and in 16 PLTP variants created by replacing 12 charged amino acids by site-directed mutagenesis. The data were analyzed in relation to the structure of a member of the same gene family, bactericidal/permeability-increasing protein, which is a boomerang-shaped molecule containing two symmetrical, hydrophobic pockets that bind phospholipid molecules. When expressed in COS-7 cells, wild-type and all mutant PLTPs accumulated intracellularly to nearly the same extent. Relative to wild-type PLTP, substitution(s) for amino acids with a lateral position totally exposed to the solvent produced reductions in transfer activity proportional to the reductions in the level of HDL binding. Variants containing substitutions for charged amino acids on the concave surface of PLTP did not affect binding to HDL or specific transfer activity. A mutation in the C-terminal pocket (E270R) led to a decrease in both the specific transfer activity and the level of binding to HDLs, whereas mutations in the N-terminal pocket (R25E and D231R) resulted in a large decrease in specific transfer activity without affecting HDL binding. The data support a model of transfer in which N- and C-terminal pockets have different roles in HDL binding and transfer activity. The N-terminal pocket may be critical to PLTP transfer activity but may have no involvement in binding to lipoproteins, whereas amino acid substitutions in the C-terminal pocket might reduce PLTP activity by decreasing PLTP's affinity for HDLs.     

Determination of human plasma phospholipid transfer protein mass and activity

Plasma phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism. PLTP is an 80-kDa glycoprotein that is expressed/secreted by a wide variety of tissues including lung, liver, adipose tissue, brain, and muscle. PLTP mediates a net transfer of phospholipids between vesicles and plasma HDLs. It also generates from small HDL particles large fused HDL particles with a concomitant formation of small lipid-poor apolipoprotein (apo) A-I-containing particles which are thought to act as the primary acceptors of cell-derived cholesterol from peripheral tissue macrophages. Another important function of PLTP is connected to lipolysis. Its role in the transfer of surface remnants from triglyceride-rich particles, very-low-density lipoproteins, and chylomicrons, to HDL is of importance for the maintenance of HDL levels. Recent observations from our laboratory have demonstrated that in circulation two forms of PLTP are present, one catalytically active (high-activity form, HA-PLTP) and the other a low-activity form (LA-PLTP). In view of the likely relevancy of PLTP in human health and disease, reliable and accurate methods for measuring plasma/serum PLTP activity and concentration are required. In this chapter, two radiometric PLTP activity assays are described: (i) exogenous, lipoprotein-independent phospholipid transfer assay and (ii) endogenous, lipoprotein-dependent phospholipid transfer assay. In addition, an ELISA method for quantitation of serum/plasma total PLTP mass as well as HA-PLTP and LA-PLTP mass is reported in detail.     

Structural basis of the lipid transfer mechanism of phospholipid transfer protein (PLTP)

Human phospholipid transfer protein (PLTP) mediates the transfer of phospholipids among atheroprotective high-density lipoproteins (HDL) and atherogenic low-density lipoproteins (LDL) by an unknown mechanism. Delineating this mechanism would represent the first step towards understanding PLTP-mediated lipid transfers, which may be important for treating lipoprotein abnormalities and cardiovascular disease. Here, using various electron microscopy techniques, PLTP is revealed to have a banana-shaped structure similar to cholesteryl ester transfer protein (CETP). We provide evidence that PLTP penetrates into the HDL and LDL surfaces, respectively, and then forms a ternary complex with HDL and LDL. Insights into the interaction of PLTP with lipoproteins at the molecular level provide a basis to understand the PLTP-dependent lipid transfer mechanisms for dyslipidemia treatment.     

ApoE-containing high density lipoproteins and phospholipid transfer protein activity increase in patients with a systemic inflammatory response

High density lipoproteins (HDL) mediate reverse cholesterol transport as well as the clearance of oxidation products or inflammatory mediators, thereby contributing to tissue integrity. The decrease in HDL in inflammation has been attributed to decreased lecithin:cholesterol acyltransferase activity, whereas the role of phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein has not been analyzed in detail. We have studied the activities of HDL-modifying proteins and the heterogeneity of HDL in healthy control subjects and three groups of postsurgery patients: no bacterial infection (group 1), bacterial focus and systemic inflammatory response (group 2), and severe sepsis (group 3). For all patients, a decrease in total HDL could be demonstrated, with a loss of mainly large, apolipoprotein A-I (apoA-I) HDL particles, an almost total loss of apoC-I, and an increase in apoE HDL (200-500 kDa), which did not contain significant amounts of apoA-I, apoA-II, or apoC-I. PLTP activity was increased in patients of groups 2 and 3, paralleled by a redistribution of PLTP into a population of small (120- to 200-kDa) particles, probably representing PLTP homodimers or lipid-complexed PLTP.In summary, the increase in apoE HDL and PLTP activity may improve the delivery of energy substrates and phospholipids to tissues that must maintain cellular membrane homeostasis under conditions of inflammatory stress.     

Plasma phospholipid transfer activity is essential for increased atherogenesis in PLTP transgenic mice: a mutation-inactivation study

Plasma phospholipid transfer protein (PLTP) interacts with HDL particles and facilitates the transfer of phospholipids from triglyceride (TG)-rich lipoproteins to HDL. Overexpressing human PLTP in mice increases the susceptibility to atherosclerosis. In human plasma, high-active and low-active forms of PLTP exist. To elucidate the contribution of phospholipid transfer activity to changes in lipoprotein metabolism and atherogenesis, we developed mice expressing mutant PLTP, still able to associate with HDL but lacking phospholipid transfer activity. In mice heterozygous for the LDL receptor, effects of the mutant and normal human PLTP transgene (mutPLTP tg and PLTP tg, respectively) were compared. In PLTP tg mice, plasma PLTP activity was increased 2.9-fold, resulting in markedly reduced HDL lipid levels. In contrast, in mutPLTP tg mice, lipid levels were not different from controls. Furthermore, hepatic VLDL-TG secretion was stimulated in PLTP tg mice, but not in mutPLTP tg mice. When mice were fed a cholesterol-enriched diet, atherosclerotic lesion size in PLTP tg mice was increased more than 2-fold compared with control mice, whereas in mutPLTP tg mice, there was no change. Our findings demonstrate that PLTP transfer activity is essential for the development of atherosclerosis in PLTP transgenic mice, identifying PLTP activity as a possible target to prevent atherogenesis, independent of plasma PLTP concentration.     

Avian phospholipid transfer protein causes HDL conversion without affecting cholesterol efflux from macrophages

Circulatory phospholipid transfer protein (PLTP) has two major functions: 1) transfer of phospholipids towards HDL particles; and 2) modulation of HDL size and composition via the HDL conversion process. In the laying hen (Gallus gallus), the massive oocyte-targeted lipid flow is achieved through the concerted actions of lipases, lipid transfer proteins, and relatives of the LDL receptor family. The aim of the study was to gain insights into the structure and functions of chicken PLTP. The results demonstrate that PLTP is highly conserved from chicken to mammals, as (i) chicken PLTP is associated with plasma HDL; (ii) it clearly possesses phospholipid transfer activity; (iii) it is inactivated at + 58 °C; and (iv) it mediates conversion of avian and human HDL into small preβ-mobile HDL and large fused α-mobile HDL particles. Our data show that HDL from different chicken models is similar in chemical and physical properties to that of man based on PLTP activity, cholesterol efflux, and HDL conversion assays. In contrast to mammals, PLTP-facilitated HDL remodeling did not enhance cholesterol efflux efficiency of chicken HDL particles.     

A hydrophobic cluster at the surface of the human plasma phospholipid transfer protein is critical for activity on high density lipoproteins

The plasma phospholipid transfer protein (PLTP) belongs to the lipid transfer/lipopolysaccharide binding protein (LT/LBP) family, together with the cholesteryl ester transfer protein, the lipopolysaccharide binding protein (LBP) and the bactericidal permeability increasing protein (BPI). In the present study, we used the crystallographic data available for BPI to build a three-dimensional model for PLTP. Multiple sequence alignment suggested that, in PLTP, a cluster of hydrophobic residues substitutes for a cluster of positively charged residues found on the surface of LBP and BPI, which is critical for interaction with lipopolysaccharides. According to the PLTP model, these hydrophobic residues are situated on an exposed hydrophobic patch at the N-terminal tip of the molecule. To assess the role of this hydrophobic cluster for the functional activity of PLTP, single point alanine mutants were engineered. Phospholipid transfer from liposomes to high density lipoprotein (HDL) by the W91A, F92A, and F93A PLTP mutants was drastically reduced, whereas their transfer activity toward very low density lipoprotein and low density lipoprotein did not change. The HDL size conversion activity of the mutants was reduced to the same extent as the PLTP transfer activity toward HDL. Based on these results, we propose that a functional solvent-exposed hydrophobic cluster in the PLTP molecule specifically contributes to the PLTP transfer activity on HDL substrates.     

Phospholipid transfer is a prerequisite for PLTP-mediated HDL conversion

Phospholipid transfer protein (PLTP) is an important regulator of high-density lipoprotein (HDL) metabolism. The two main functions of PLTP are transfer of phospholipids between lipoprotein particles and modulation of HDL size and composition in a process called HDL conversion. These PLTP-mediated processes are physiologically important in the transfer of surface remnants from lipolyzed triglyceride-rich lipoproteins to nascent HDL particles and in the generation of prebeta-HDL, the initial acceptor of excess peripheral cell cholesterol. The aim of the study presented here was to investigate the interrelationship between the two functions of PLTP. Plasma PLTP was chemically modified using diethylpyrocarbonate or ethylmercurithiosalicylate. The modified proteins displayed a dose-dependent decrease in phospholipid transfer activity and a parallel decrease in the ability to cause HDL conversion. Two recombinant PLTP mutant proteins, defective in phospholipid transfer activity due to a mutation in the N-terminal lipid-binding pocket, were produced, isolated, and incubated together with radioactively labeled HDL(3). HDL conversion was analyzed using three methods: native gradient gel electrophoresis, ultracentrifugation, and crossed immunoelectrophoresis. The results demonstrate that the mutant proteins (i) are able to induce only a modest increase in HDL particle size compared to the wild-type protein, (ii) are unable to release apoA-I from HDL(3), and (iii) do not generate prebeta-mobile particles following incubation with HDL(3). These data suggest that phospholipid transfer is a prerequisite for HDL conversion and demonstrate the close interrelationship between the two main activities of PLTP.     

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.     

Plasma phospholipid transfer protein-mediated reactions are impaired by hypochlorite-modification of high density lipoprotein

The two main functions of phospholipid transfer protein (PLTP) are the transfer of phospholipids between plasma lipoproteins and the conversion of high density lipoprotein (HDL), where prebeta-HDL particles are generated. HDL is considered an anti-atherogenic lipoprotein due to its function in the reverse cholesterol transport, where prebeta-HDL accepts cellular membrane cholesterol from peripheral tissues. However, the anti-atherogenic properties of native HDL may be abolished by oxidation/modification. Hypochlorous acid/hypochlorite (HOCl/OCl-)-a potent oxidant generated in vivo only by the myeloperoxidase-H2O2-chloride system of activated phagocytes-alters the physiological properties of HDL by generating a pro-atherogenic lipoprotein particle. Therefore, we have studied the effect of HOCl on the function of HDL subclass 3 (HDL3) and triglyceride-enriched HDL3 (TG-HDL3) in PLTP-mediated processes in vitro. Modification of HDL3 and TG-HDL3 with increasing HOCl concentrations (oxidant:lipoprotein molar ratio between 25:1 and 200:1) decreased the capacity of the corresponding lipoprotein particles to accept phospholipids. Although binding of PLTP to unmodified and HOCl-modified lipoprotein particles was similar, the degree of PLTP-mediated HDL conversion was decreased upon HOCl oxidation. PLTP released apolipoprotein A-I (apoA-I) from HOCl-modified HDL3, but the particles formed displayed no prebeta-mobility. Based on these findings, we conclude that the substrate properties of HOCl-modified HDL3 and TG-HDL3 in PLTP-mediated processes are impaired, which indicates that the anti-atherogenic properties of HDL are impaired.     

Distribution of phospholipid transfer protein in human plasma: presence of two forms of phospholipid transfer protein, one catalytically active and the other inactive

Plasma phospholipid transfer protein (PLTP) plays an important role in the maintenance of plasma high-density lipoprotein (HDL) content and remodeling of HDL in the circulation. In the present study we have used different fractionation methods to investigate the distribution of PLTP in human plasma. A novel enzyme-linked immunosorbent assay developed during the study allowed for simultaneous assessment of both PLTP mass and activity in the fractions obtained. Size-exclusion chromatography and plasma fractionation by nondenaturing polyacrylamide gel electrophoresis (PAGE) yielded similar results demonstrating that PLTP associates in native plasma with two distinct particle populations, while ultracentrifugation with high salt leads to detachment of PLTP from lipoprotein particles and loss of a majority of its phospholipid transfer activity. Interestingly, analysis of the size-exclusion chromatography fractions demonstrated that PLTP exists in the circulation as an active population that elutes in the position of HDL corresponding to an average molecular mass of 160+/-40 kDa and an inactive form with an average mass of 520+/-120 kDa. The inactive fraction containing approximately 70% of the total PLTP protein eluted between HDL and low density lipoprotein (LDL). Thus, the two PLTP pools are associated with different types of lipoprotein particles, suggesting that the PLTP activity in circulation is modulated by the plasma lipoprotein profile and lipid composition.     

Remodeling of apolipoprotein E-containing spherical reconstituted high density lipoproteins by phospholipid transfer protein

Phospholipid transfer protein (PLTP) transfers phospholipids between HDL and other lipoproteins in plasma. It also remodels spherical, apolipoprotein A-I (apoA-I)-containing HDL into large and small particles in a process involving the dissociation of lipid-free/lipid-poor apoA-I. ApoE is another apolipoprotein that is mostly associated with large, spherical HDL that do not contain apoA-I. Three isoforms of apoE have been identified in human plasma: apoE2, apoE3, and apoE4. This study investigates the remodeling of spherical apoE-containing HDL by PLTP and the ability of PLTP to transfer phospholipids between apoE-containing HDL and phospholipid vesicles. Spherical reconstituted high density lipoproteins (rHDL) containing apoA-I [(A-I)rHDL], apoE2 [(E2)rHDL], apoE3 [(E3)rHDL], or apoE4 [(E4)rHDL] as the sole apolipoprotein were prepared by incubating discoidal rHDL with low density lipoproteins and lecithin:cholesterol acyltransferase. PLTP remodeled the spherical, apoE-containing rHDL into large and small particles without the dissociation of apoE. The PLTP-mediated remodeling of apoE-containing rHDL was more extensive than that of (A-I)rHDL. PLTP transferred phospholipids from small unilamellar vesicles to apoE-containing rHDL in an isoform-dependent manner, but at a rate slower than that for spherical (A-I)rHDL. It is concluded that apoE enhances the capacity of PLTP to remodel HDL but reduces the ability of HDL to participate in PLTP-mediated phospholipid transfers.     

Increased risk of atherosclerosis by elevated plasma levels of phospholipid transfer protein

Plasma phospholipid transfer protein (PLTP) is thought to be involved in the remodeling of high density lipoproteins (HDL), which are atheroprotective. It is also involved in the metabolism of very low density lipoproteins (VLDL). Hence, PLTP is thought to be an important factor in lipoprotein metabolism and the development of atherosclerosis. We have overexpressed PLTP in mice heterozygous for the low density lipoprotein (LDL) receptor, a model for atherosclerosis. We show that increased PLTP activity results in a dose-dependent decrease in HDL, and a moderate stimulation of VLDL secretion (相似文献   

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.     

PLTP secreted by HepG2 cells resembles the high-activity PLTP form in human plasma

Plasma phospholipid transfer protein (PLTP) is an important regulator of plasma HDL levels and HDL particle distribution. PLTP is present in plasma in two forms, one with high and the other with low phospholipid transfer activity. We have used the human hepatoma cell line, HepG2, as a model to study PLTP secreted from hepatic cells. PLTP activity was secreted by the cells into serum-free culture medium as a function of time. However, modification of a previously established ELISA assay to include a denaturing sample pretreatment with the anionic detergent sodium dodecyl sulphate was required for the detection of the secreted PLTP protein. The HepG2 PLTP could be enriched by Heparin-Sepharose affinity chromatography and eluted in size-exclusion chromatography at a position corresponding to the size of 160 kDa. PLTP coeluted with apolipoprotein E (apoE) but not with apoB-100 or apoA-I. A portion of PLTP was retained by an anti-apoE immunoaffinity column together with apoE, suggesting an interaction between these two proteins. Furthermore, antibodies against apoE but not those against apoB-100 or apoA-I were capable of inhibiting PLTP activity. These results show that the HepG2-derived PLTP resembles in several aspects the high-activity form of PLTP found in human plasma.     

PLTP deficiency improves the anti-inflammatory properties of HDL and reduces the ability of LDL to induce monocyte chemotactic activity

We reported that phospholipid transfer protein (PLTP) deficiency decreased atherosclerosis in mouse models. Because the decreased atherosclerosis was accompanied by a significant decrease in plasma HDL levels, we examined the properties of PLTP knockout (PLTP0) HDL and tested its ability to prevent LDL-induced monocyte chemotactic activity in human artery wall cell cocultures. We isolated HDL and LDL from LDL receptor knockout/PLTP knockout (LDLr0/PLTP0) mice and from apolipoprotein B transgenic (apoBTg)/PLTP0 mice as well as their controls. PLTP0 HDL was relatively rich in protein and depleted in phosphatidylcholine. Turnover studies revealed a 3.5- to 4.0-fold increase in the turnover of protein and cholesteryl ester in HDL from PLTP0 mice compared with control mice. The ability of HDL from LDLr0/PLTP0 and apoBTg/PLTP0 mice to prevent the induction of monocyte chemotactic activity in human artery wall cell cocultures exposed to human LDL was dramatically better than that in controls. Moreover, LDL from PLTP0 mice was markedly resistant to oxidation and induced significantly less monocyte chemotactic activity compared with that in controls. In vitro, PLTP0 HDL removed significantly more oxidized phospholipids from LDL than did control HDL. We conclude that PLTP deficiency improves the anti-inflammatory properties of HDL in mice and reduces the ability of LDL to induce monocyte chemotactic activity.     

PLTP activity in premenopausal women. Relationship with lipoprotein lipase, HDL, LDL, body fat, and insulin resistance

Plasma phospholipid transfer protein (PLTP) is thought to play a major role in the facilitated transfer of phospholipids between lipoproteins and in the modulation of high density lipoprotein (HDL) particle size and composition. However, little has been reported concerning the relationships of PLTP with plasma lipoprotein parameters, lipolytic enzymes, body fat distribution, insulin, and glucose in normolipidemic individuals, particularly females. In the present study, 50 normolipidemic healthy premenopausal females were investigated. The relationships between the plasma PLTP activity and selected variables were assessed. PLTP activity was significantly and positively correlated with low density lipoprotein (LDL) cholesterol (r(s) = 0.53), apoB (r(s) = 0.44), glucose (r(s) = 0.40), HDL cholesterol (r(s) = 0.38), HDL(3) cholesterol (r(s) = 0.37), lipoprotein lipase activity (r(s) = 0.36), insulin (r(s) = 0.33), subcutaneous abdominal fat (r(s) = 0.36), intra-abdominal fat (r(s) = 0.29), and body mass index (r(s) = 0.29). HDL(2) cholesterol, triglyceride, and hepatic lipase were not significantly related to PLTP activity. As HDL(2) can be decreased by hepatic lipase and hepatic lipase is increased in obesity with increasing intra-abdominal fat, the participants were divided into sub-groups of non-obese (n = 35) and obese (n = 15) individuals and the correlation of PLTP with HDL(2) cholesterol was re-examined. In the non-obese subjects, HDL(2) cholesterol was found to be significantly and positively related to PLTP activity (r(s) = 0.44). Adjustment of the HDL(2) values for the effect of hepatic lipase activity resulted in a significant positive correlation between PLTP and HDL(2) (r(s) = 0.41), indicating that the strength of the relationship between PLTP activity and HDL(2) can be reduced by the opposing effect of hepatic lipase on HDL(2) concentrations. We conclude that PLTP-facilitated lipid transfer activity is related to HDL and LDL metabolism, as well as lipoprotein lipase activity, adiposity, and insulin resistance.