Phospholipid transfer protein gene knock-out mice have low high density lipoprotein levels, due to hypercatabolism, and accumulate apoA-IV-rich lamellar lipoproteins

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

New therapeutic horizons for plasma phospholipid transfer protein (PLTP): Targeting endotoxemia, infection and sepsis

Phospholipid Transfer Protein (PLTP) transfers amphiphilic lipids between circulating lipoproteins and between lipoproteins, cells and tissues. Indeed, PLTP is a major determinant of the plasma levels, turnover and functionality of the main lipoprotein classes: very low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL). To date, most attention has been focused on the role of PLTP in the context of cardiometabolic diseases, with additional insights in neurodegenerative diseases and immunity. Importantly, beyond its influence on plasma triglyceride and cholesterol transport, PLTP plays a key role in the modulation of the immune response, with immediate relevance to a wide range of inflammatory diseases including bacterial infection and sepsis. Indeed, emerging evidence supports the role of PLTP, in the context of its association with lipoproteins, in the neutralization and clearance of bacterial lipopolysaccharides (LPS) or endotoxins. LPS are amphipathic molecules originating from Gram-negative bacteria which harbor major pathogen-associated patterns, triggering an innate immune response in the host. Although the early inflammatory reaction constitutes a key step in the anti-microbial defense of the organism, it can lead to a dysregulated inflammatory response and to hemodynamic disorders, organ failure and eventually death. Moreover, and in addition to endotoxemia and acute inflammation, small amounts of LPS in the circulation can induce chronic, low-grade inflammation with long-term consequences in several metabolic disorders such as atherosclerosis, obesity and diabetes. After an updated overview of the role of PLTP in lipid transfer, lipoprotein metabolism and related diseases, current knowledge of its impact on inflammation, infection and sepsis is critically appraised. Finally, the relevance of PLTP as a new player and novel therapeutic target in the fight against inflammatory diseases is considered.

IL-2Rβγ signalling in lymphocytes promotes systemic inflammation and reduces plasma cholesterol in atherosclerotic mice

BACKGROUND AND AIMS

The relationship between inflammation and lipid metabolism is complex and bidirectional. Lymphocyte-driven inflammation has been shown to modulate both atherosclerotic plaque development and cholesterol levels, but the mechanisms are incompletely understood.

METHODS

The cardiometabolic effects of IL-2Rβγ signalling in atherosclerotic Apoe^-/- mice were investigated by treatment with an agonistic IL-2Rβγ-targeting IL-2/anti-IL-2 complex or a monoclonal anti-CD122 (IL-2Rβ) blocking antibody.

RESULTS

Administration of IL-2Rβγ agonistic IL-2/anti-IL-2 complexes to Apoe^-/- mice augmented opposing arms of the adaptive immune system. Expansion of effector/memory T cells and increased levels of circulating pro-inflammatory cytokines were observed along with elevated levels of regulatory T cells and IL-10. Notably, IL-2/anti-IL-2 treatment did not affect plaque size but decreased levels of plasma cholesterol. The cholesterol lowering effect of IL-2Rβγ agonism was not affected by anti-CD8 or anti-NK1.1 depleting antibody treatment but was contingent on the presence of adaptive immunity. Expression of multiple liver X receptor (LXR)-related genes, including Pltp and Srebp1c in the liver, was decreased by IL-2/anti-IL-2 treatment. Although IL-2Rβγ agonism lowered cholesterol levels, blocking IL-2Rβγ signalling using an anti-CD122 monoclonal antibody did not impact cholesterol levels or plaque burden in Apoe^-/- mice.

CONCLUSIONS

Elevated IL-2Rβγ signalling results in activation of both inflammatory and regulatory lymphocytes with a net zero effect on atherosclerosis and decreased plasma cholesterol levels. Changes in cholesterol levels were associated with reductions in hepatic LXR-related gene expression. Further studies are needed to investigate the clinical significance of IL-2 mediated modulation of hepatic LXR signalling in inflammatory disorders.

The Role of Phospholipid Transfer Protein in the Development of Atherosclerosis

PURPOSE OF REVIEW

Phospholipid transfer protein (PLTP), a member of lipid transfer protein family, is an important protein involved in lipid metabolism in the circulation. This article reviews recent PLTP research progresses, involving lipoprotein metabolism and atherogenesis.

RECENT FINDINGS

PLTP activity influences atherogenic and anti-atherogenic lipoprotein levels. Human serum PLTP activity is a risk factor for human cardiovascular disease and is an independent predictor of all-cause mortality. PLTP deficiency reduces VLDL and LDL levels and attenuates atherosclerosis in mouse models, while PLTP overexpression exerts an opposite effect. Both PLTP deficiency and overexpression result in reduction of HDL which has different size, inflammatory index, and lipid composition. Moreover, although both PLTP deficiency and overexpression reduce cholesterol efflux capacity, but this effect has no impact in macrophage reverse cholesterol transport in mice. Furthermore, PLTP activity is related with metabolic syndrome, thrombosis, and inflammation. PLTP could be target for the treatment of dyslipidemia and atherosclerosis, although some potential off-target effects should be noted.

Impact of Phospholipid Transfer Protein in Lipid Metabolism and Cardiovascular Diseases

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

Phospholipid transfer protein and alpha-1 antitrypsin regulate Hck kinase activity during neutrophil degranulation

Excessive neutrophil degranulation is a common feature of many inflammatory disorders, including alpha-1 antitrypsin (AAT) deficiency. Our group has demonstrated that phospholipid transfer protein (PLTP) prevents neutrophil degranulation but serine proteases, which AAT inhibits, cleave PLTP in diseased airways. We propose to identify if airway PLTP activity can be restored by AAT augmentation therapy and how PLTP subdues degranulation of neutrophils in AAT deficient subjects. Airway PLTP activity was lower in AAT deficient patients but elevated in the airways of patients on augmentation therapy. Functional AAT protein (from PiMM homozygotes) prevented PLTP cleavage unlike its mutated ZZ variant (PiZZ). PLTP lowered leukotriene B4 induced degranulation of primary, secondary and tertiary granules from neutrophils from both groups (n = 14/group). Neutrophils isolated from Pltp knockout mice have enhance neutrophil degranulation. Both AAT and PLTP reduced neutrophil degranulation and superoxide production, possibly though their inhibition of the Src tyrosine kinase, Hck. Src kinase inhibitors saracatinib and dasatinib reduced neutrophil degranulation and superoxide production. Therefore, AAT protects PLTP from proteolytic cleavage and both AAT and PLTP mediate degranulation, possibly via Hck tyrosine kinase inhibition. Deficiency of AAT could contribute to reduced lung PLTP activity and elevated neutrophil signaling associated with lung disease.

Unravelling HDL-Looking beyond the Cholesterol Surface to the Quality Within

High-density lipoprotein (HDL) particles have experienced a turbulent decade of falling from grace with widespread demotion from the most-sought-after therapeutic target to reverse cardiovascular disease (CVD), to mere biomarker status. HDL is slowly emerging from these dark times due to the HDL flux hypothesis wherein measures of HDL cholesterol efflux capacity (CEC) are better predictors of reduced CVD risk than static HDL-cholesterol (HDL-C) levels. HDL particles are emulsions of metabolites, lipids, protein, and microRNA (miR) built on the backbone of Apolipoprotein A1 (ApoA1) that are growing in their complexity due to the higher sensitivity of the respective “omic” technologies. Our understanding of particle composition has increased dramatically within this era and has exposed how our understanding of these particles to date has been oversimplified. Elucidation of the HDL proteome coupled with the identification of specific miRs on HDL have highlighted the “hormonal” characteristics of HDL in that it carries and delivers messages systemically. HDL can dock to most peripheral cells via its receptors, including SR-B1, ABCA1, and ABCG1, which may be a critical step for facilitating HDL-to-cell communication. The composition of HDL particles is, in turn, altered in numerous disease states including diabetes, auto-immune disease, and CVD. The consequence of changes in composition, however, on subsequent biological activities of HDL is currently poorly understood and this is an important avenue for the field to explore in the future. Improving HDL particle quality as opposed to HDL quantity may, in turn, prove a more beneficial investment to reduce CVD risk.

Phospholipid transfer protein: its impact on lipoprotein homeostasis and atherosclerosis

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

Plasma cholesterol level determines in vivo prion propagation

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases with an urgent need for therapeutic and prophylactic strategies. At the time when the blood-mediated transmission of prions was demonstrated, in vitro studies indicated a high binding affinity of the scrapie prion protein (PrP^Sc) with apoB-containing lipoproteins, i.e., the main carriers of cholesterol in human blood. The aim of the present study was to explore the relationship between circulating cholesterol-containing lipoproteins and the pathogenicity of prions in vivo. We showed that, in mice with a genetically engineered deficiency for the plasma lipid transporter, phospholipid transfer protein (PLTP), abnormally low circulating cholesterol concentrations were associated with a significant prolongation of survival time after intraperitoneal inoculation of the 22L prion strain. Moreover, when circulating cholesterol levels rose after feeding PLTP-deficient mice a lipid-enriched diet, a significant reduction in survival time of mice together with a marked increase in the accumulation rate of PrP^Sc deposits in their brain were observed. Our results suggest that the circulating cholesterol level is a determinant of prion propagation in vivo and that cholesterol-lowering strategies might be a successful therapeutic approach for patients suffering from prion diseases.

Mouse models of disturbed HDL metabolism

High-density lipoprotein (HDL) is considered to be an anti-atherogenic lipoprotein moiety. Generation of genetically modified (total body and tissue-specific knockout) mouse models has significantly contributed to our understanding of HDL function. Here we will review data from knockout mouse studies on the importance of HDL's major alipoprotein apoA-I, the ABC transporters A1 and G1, lecithin:cholesterol acyltransferase, phospholipid transfer protein, and scavenger receptor BI for HDL's metabolism and its protection against atherosclerosis in mice. The initial generation and maturation of HDL particles as well as the selective delivery of its cholesterol to the liver are essential parameters in the life cycle of HDL. Detrimental atherosclerosis effects observed in response to HDL deficiency in mice cannot be solely attributed to the low HDL levels per se, as the low HDL levels are in most models paralleled by changes in non-HDL-cholesterol levels. However, the cholesterol efflux function of HDL is of critical importance to overcome foam cell formation and the development of atherosclerotic lesions in mice. Although HDL is predominantly studied for its atheroprotective action, the mouse data also suggest an essential role for HDL as cholesterol donor for steroidogenic tissues, including the adrenals and ovaries. Furthermore, it appears that a relevant interaction exists between HDL-mediated cellular cholesterol efflux and the susceptibility to inflammation, which (1) provides strong support for the novel concept that inflammation and metabolism are intertwining biological processes and (2) identifies the efflux function of HDL as putative therapeutic target also in other inflammatory diseases than atherosclerosis.

Adipocyte phospholipid transfer protein and lipoprotein metabolism

OBJECTIVE

Phospholipid transfer protein (PLTP) is highly expressed in adipose tissues. Thus, the effect of adipose tissue PLTP on plasma lipoprotein metabolism was examined.

APPROACH AND RESULTS

We crossed PLTP-Flox-ΔNeo and adipocyte protein 2 (aP2)-Cre recombinase (Cre) transgenic mice to create PLTP-Flox-ΔNeo/aP2-Cre mice that have a 90 and a 60% reduction in PLTP mRNA in adipose tissue and macrophages, respectively. PLTP ablation resulted in a significant reduction in plasma PLTP activity (22%), high-density lipoprotein-cholesterol (21%), high-density lipoprotein-phospholipid (20%), and apolipoprotein A-I (33%) levels, but had no effect on nonhigh-density lipoprotein levels in comparison with those of PLTP-Flox-ΔNeo controls. To eliminate possible effects of PLTP ablation by macrophages, we lethally irradiated PLTP-Flox-ΔNeo/aP2-Cre mice and PLTP-Flox-ΔNeo mice, and then transplanted wild-type mouse bone marrow into them to create wild-type→PLTP-Flox-ΔNeo/aP2-Cre and wild-type→PLTP-Flox-ΔNeo mice. Thus, we constructed a mouse model (wild-type→PLTP-Flox-ΔNeo/aP2-Cre) with PLTP deficiency in adipocytes but not in macrophages. These knockout mice also showed significant decreases in plasma PLTP activity (19%) and cholesterol (18%), phospholipid (17%), and apolipoprotein A-I (26%) levels. To further investigate the mechanisms behind the reduction in plasma apolipoprotein A-I and high-density lipoprotein lipids, we measured apolipoprotein A-I-mediated cholesterol efflux in adipose tissue explants and found that endogenous and exogenous PLTP significantly increased cholesterol efflux from the explants.

CONCLUSIONS

Adipocyte PLTP plays a small but significant role in plasma PLTP activity and promotes cholesterol efflux from adipose tissues.

Phospholipid transfer protein destabilizes mouse atherosclerotic plaque

OBJECTIVE

Phospholipid transfer protein (PLTP) accelerates the development of atherosclerosis in mouse models. We examined the role of PLTP in atherosclerotic plaque stability.

APPROACH AND RESULTS

We prepared apolipoprotein E and PLTP double-knockout (PLTP(-/-)ApoE(-/-)) mice. PLTP deficiency significantly decreased lesion size and reduced monocyte/macrophage infiltration, as well as macrophage apoptosis in lesion areas. Moreover, it increased fibrous content in plaques, which suggests that PLTP may affect atherosclerotic plaque stability. Importantly, PLTP overexpression mediated by adenovirus had the reverse effect. It promoted the accumulation of reactive oxygen species in macrophages, which could lead to cell apoptosis and increased the production of inflammatory cytokines and chemokines. PLTP overexpression could promote receptor-interacting protein 3 recruitment of macrophages in cytoplasm, which could induce reactive oxygen species, thus inducing atherogenesis.

CONCLUSIONS

PLTP plays an important role in modulating the stability of atherosclerotic plaques. The receptor-interacting protein 3- reactive oxygen species signal pathway could be involved in this PLTP-mediated process.

The role of sphingosine-1-phosphate in endothelial barrier function

Loss of endothelial barrier function is implicated in the etiology of metastasis, atherosclerosis, sepsis and many other diseases. Studies suggest that sphingosine-1-phosphate (S1P), particularly HDL-bound S1P (HDL-S1P) is essential for endothelial barrier homeostasis and that HDL-S1P may be protective against the loss of endothelial barrier function in disease. This review summarizes evidence providing mechanistic insights into how S1P maintains endothelial barrier function, highlighting the recent findings that implicate the major S1P carrier, HDL, in the maintenance of the persistent S1P-signaling needed to maintain endothelial barrier function. We review the mechanisms proposed for HDL maintenance of persistent S1P-signaling, the evidence supporting these mechanisms and the remaining fundamental questions.

A computational model for the analysis of lipoprotein distributions in the mouse: translating FPLC profiles to lipoprotein metabolism

Disturbances of lipoprotein metabolism are recognized as indicators of cardiometabolic disease risk. Lipoprotein size and composition, measured in a lipoprotein profile, are considered to be disease risk markers. However, the measured profile is a collective result of complex metabolic interactions, which complicates the identification of changes in metabolism. In this study we aim to develop a method which quantitatively relates murine lipoprotein size, composition and concentration to the molecular mechanisms underlying lipoprotein metabolism. We introduce a computational framework which incorporates a novel kinetic model of murine lipoprotein metabolism. The model is applied to compute a distribution of plasma lipoproteins, which is then related to experimental lipoprotein profiles through the generation of an in silico lipoprotein profile. The model was first applied to profiles obtained from wild-type C57Bl/6J mice. The results provided insight into the interplay of lipoprotein production, remodelling and catabolism. Moreover, the concentration and metabolism of unmeasured lipoprotein components could be determined. The model was validated through the prediction of lipoprotein profiles of several transgenic mouse models commonly used in cardiovascular research. Finally, the framework was employed for longitudinal analysis of the profiles of C57Bl/6J mice following a pharmaceutical intervention with a liver X receptor (LXR) agonist. The multifaceted regulatory response to the administration of the compound is incompletely understood. The results explain the characteristic changes of the observed lipoprotein profile in terms of the underlying metabolic perturbation and resultant modifications of lipid fluxes in the body. The Murine Lipoprotein Profiler (MuLiP) presented here is thus a valuable tool to assess the metabolic origin of altered murine lipoprotein profiles and can be applied in preclinical research performed in mice for analysis of lipid fluxes and lipoprotein composition.

Comparison of vegetarian diets and omnivorous diets on plasma level of HDL-c: a meta-analysis

Low plasma level of high density lipoprotein cholesterol (HDL-c) was an independent risk factor for cardio vascular disorder, and associated with poor outcomes in pulmonary arterial hypertension. To compare the effects of vegetarian diets and omnivorous diets on HDL-c in plasma, we identified cross-sectional and cohort studies related to HDL-c listed on PubMed and ISI Web of Knowledge as well as the corresponding references (until Nov, 2013). Twelve studies with a total of 4177 individuals were selected for meta-analysis. This meta-analysis indicates that vegetarian diets did not alter plasma HDL-c concentrations, as it wasn’t initially expected by the authors [Standardized Mean Difference (SMD) = 0.02 mmol/l; 95% confidence interval (CI): −0.19 to 0.22 mmol/l]. In Asia and Latin America countries, no significant differences in HDL-c levels between vegetarians and omnivores were found (SMD = −0.09 mmol/l; 95% CI: −0.43 to 0.25 mmol/l). In Europe and North America countries, the plasma level of HDL-c was also not different between the two diets (SMD = 0.09 mmol/l; 95% CI: −0.19 to 0.36 mmol/l). In light of this meta-analysis, we conclude that there is no evidence that plasma HDL-c levels differs in vegetarians and omnivores, even after adjusting for cultural circumstances.

Newer therapeutic strategies to alter high-density lipoprotein level and function

Measurements of low levels of high-density lipoprotein (HDL) cholesterol have been identified as a risk factor for premature coronary artery disease, however, to date, current pharmacologic approaches for raising HDL have provided little benefit, if at all, in reducing cardiovascular outcomes. It has been shown that HDL can modify many aspects of plaque pathogenesis. Its most established role is in reverse cholesterol transportation, but HDL can also affect oxidation, inflammation, cellular adhesion, and vasodilatation. Considering these potential benefits of HDL, newer treatments have been developed to modify HDL activity, which include the use of oral cholesteryl ester transfer protein inhibitors, apolipoprotein (apo)A-I infusions, apoA-I mimetics, drugs to increase apoA-I synthesis, and agonists of the liver X receptor. These new therapies are reviewed in this article.

Osbpl8 deficiency in mouse causes an elevation of high-density lipoproteins and gender-specific alterations of lipid metabolism

OSBP-related protein 8 (ORP8) encoded by Osbpl8 is an endoplasmic reticulum sterol sensor implicated in cellular lipid metabolism. We generated an Osbpl8^−/− (KO) C57Bl/6 mouse strain. Wild-type and Osbpl8KO animals at the age of 13-weeks were fed for 5 weeks either chow or high-fat diet, and their plasma lipids/lipoproteins and hepatic lipids were analyzed. The chow-fed Osbpl8KO male mice showed a marked elevation of high-density lipoprotein (HDL) cholesterol (+79%) and phospholipids (+35%), while only minor increase of apolipoprotein A-I (apoA-I) was detected. In chow-fed female KO mice a less prominent increase of HDL cholesterol (+27%) was observed, while on western diet the HDL increment was prominent in both genders. The HDL increase was accompanied by an elevated level of HDL-associated apolipoprotein E in male, but not female KO animals. No differences between genotypes were observed in lecithin:cholesterol acyltransferase (LCAT) or hepatic lipase (HL) activity, or in the fractional catabolic rate of fluorescently labeled mouse HDL injected in chow-diet fed animals. The Osbpl8KO mice of both genders displayed reduced phospholipid transfer protein (PLTP) activity, but only on chow diet. These findings are consistent with a model in which Osbpl8 deficiency results in altered biosynthesis of HDL. Consistent with this hypothesis, ORP8 depleted mouse hepatocytes secreted an increased amount of nascent HDL into the culture medium. In addition to the HDL phenotype, distinct gender-specific alterations in lipid metabolism were detected: Female KO animals on chow diet showed reduced lipoprotein lipase (LPL) activity and increased plasma triglycerides, while the male KO mice displayed elevated plasma cholesterol biosynthetic markers cholestenol, desmosterol, and lathosterol. Moreover, modest gender-specific alterations in the hepatic expression of lipid homeostatic genes were observed. In conclusion, we report the first viable OsbplKO mouse model, demonstrating a HDL elevating effect of Osbpl8 knock-out and additional gender- and/or diet-dependent impacts on lipid metabolism.

Genetic determinants of cardiometabolic risk: a proposed model for phenotype association and interaction

This review provides a translational and unifying summary of metabolic syndrome genetics and highlights evidence that genetic studies are starting to unravel and untangle origins of the complex and challenging cluster of disease phenotypes. The associated genes effectively express in the brain, liver, kidney, arterial endothelium, adipocytes, myocytes, and β cells. Progression of syndrome traits has been associated with ectopic lipid accumulation in the arterial wall, visceral adipocytes, myocytes, and liver. Thus, it follows that the genetics of dyslipidemia, obesity, and nonalcoholic fatty liver disease are central in triggering progression of the syndrome to overt expression of disease traits and have become a key focus of interest for early detection and for designing prevention and treatments. To support the "birds' eye view" approach, we provide a road-map depicting commonality and interrelationships between the traits and their genetic and environmental determinants based on known risk factors, metabolic pathways, pharmacologic targets, treatment responses, gene networks, pleiotropy, and association with circadian rhythm. Although only a small portion of the known heritability is accounted for and there is insufficient support for clinical application of gene-based prediction models, there is direction and encouraging progress in a rapidly moving field that is beginning to show clinical relevance.

Liver phospholipid transfer protein (PLTP) expression with a PLTP-null background promotes very low-density lipoprotein production in mice

It is known that plasma phospholipid transfer protein (PLTP) activity influences lipoprotein metabolism. The liver is one of the major sites of lipoprotein production and degradation, as well as of PLTP expression. To address the impact of liver-expressed PLTP on lipoprotein metabolism, we created a mouse model that expresses PLTP in the liver acutely and specifically, with a PLTP-null background. This approach in mouse model preparations can also be used universally for evaluating the function of many other genes in the liver. We found that liver PLTP expression dramatically increases plasma levels of non-high-density lipoprotein (HDL) cholesterol (2.7-fold, P < 0.0001), non-HDL phospholipid (2.5-fold, P < 0.001), and triglyceride (51%, P < 0.01), but has no significant influence on plasma HDL lipids compared with controls. Plasma apolipoprotein (apo)B levels were also significantly increased in PLTP-expressing mice (2.2-fold, P < 0.001), but those of apoA-I were not. To explore the mechanism involved, we examined the lipidation and secretion of nascent very low-density lipoprotein (VLDL), finding that liver PLTP expression significantly increases VLDL lipidation in hepatocyte microsomal lumina, and also VLDL secretion into the plasma.

CONCLUSION

It is possible to prepare a mouse model that expresses the gene of interest only in the liver, but not in other tissues. Our results suggest, for the first time, that the major function of liver PLTP is to drive VLDL production and makes a small contribution to plasma PLTP activity.

Role of phospholipid transfer protein in high-density lipoprotein- mediated reverse cholesterol transport

Reverse cholesterol transport (RCT) describes the process whereby cholesterol in peripheral tissues is transported to the liver where it is ultimately excreted in the form of bile. Given the atherogenic role of cholesterol accumulation within the vessel intima, removal of cholesterol through RCT is considered an anti-atherogenic process. The major constituents of RCT include cell membrane– bound lipid transporters, plasma lipid acceptors, plasma proteins and enzymes, and lipid receptors of liver cell membrane. One major cholesterol acceptor in RCT is high-density lipoprotein (HDL). Both the characteristics and level of HDL are critical determinants for RCT. It is known that phospholipid transfer protein (PLTP) impacts both HDL cholesterol level and biological quality of the HDL molecule. Recent data suggest that PLTP has a site-specific variation in its function. Moreover, the RCT pathway also has multiple steps both in the peripheral tissues and circulation. Therefore, PLTP may influence the RCT pathway at multiple levels. In this review, we focus on the potential role of PLTP in RCT through its impact on HDL homeostasis. The relationship between PLTP and RCT is expected to be an important area in finding novel therapies for atherosclerosis.

Chitosan oligosaccharide decreases very-low-density lipoprotein triglyceride and increases high-density lipoprotein cholesterol in high-fat-diet-fed rats

It is well known that chitosan has beneficial lipid-regulating effects, but it remains unknown whether chitosan oligosaccharide (COS), the chitosan degradation product, has the same lipid benefits. High-fat-diet-fed Wistar rats were administrated with COS by gastric gavage for three weeks. The effects of COS on lipids, lipoprotein components and lipid metabolism related protein activities were investigated. Plasma lipids level assays by an enzyme method showed that COS decreased triglyceride (TG) by 29–31%, and increased high-density lipoprotein (HDL) cholesterol by 8–11%, but did not affect low-density lipoprotein (LDL) cholesterol. Lipid distribution analysis through fast protein liquid chromatography indicated that COS significantly decreased TG content distributed in very-low-density lipoprotein (VLDL)/LDL fractions but increased cholesterol content in HDL fractions. Apolipoprotein analysis through plasma ultracentrifugation and sodium dodecyl sulfate polyacrylamide gel electrophoresis displayed that COS decreased apolipoprotein B-100 of LDL and increased apolipoprotein E of LDL and apolipoprotein B-100 of VLDL, but did not change apoA-I content of HDL particles. Lipoprotein formation associated protein determination showed that COS also increased plasma activity of lecithin cholesterol acyl transferase but not phospholipid transfer protein. The present study suggests that COS may play a beneficial role in plasma lipid regulation of rats with dyslipidemia induced by high-fat diet. The COS-decreased VLDL/LDL TG and -enhanced HDL cholesterol may be related to the upregulated activity of lecithin cholesterol acyl transferase.

Diet-induced lipid accumulation in phospholipid transfer protein-deficient mice: its atherogenicity and potential mechanism

A high saturated fat diet induces free cholesterol and phospholipid accumulation in the plasma of phospholipid transfer protein (Pltp)-deficient mice. In this study, we examined the atherogenic consequence of this phenomenon and investigated the possible mechanism(s). Pltp KO/Apoe KO mice that were fed a coconut oil-enriched high-fat diet (COD) for 7 weeks had higher plasma free cholesterol (149%), phospholipids (15%), and sphingomyelin (54%) than Apoe KO controls. In contrast to chow-fed animals, COD-fed Pltp KO/Apoe KO mice had the same atherosclerotic lesion size as that of Apoe KO mice. Similar to Pltp KO mice, plasma from COD-fed Pltp KO/Apoe KO mice contained VLDL/LDL-sized lamellar particles. Bile measurement indicated that COD-fed Pltp KO mice have 33% less hepatic cholesterol output than controls. In conclusion, COD-fed, Pltp-deficient mice are no longer protected from atherosclerosis and have impaired biliary lipid secretion, which is associated with free cholesterol and phospholipid accumulation.

Plasma PLTP activity is inversely associated with HDL-C levels

Phospholipid transfer protein (PLTP) is an important modulator of lipoprotein metabolism, including interparticle phospholipid transfer, remodeling of HDL, cholesterol and phospholipid efflux from peripheral tissues, and the production of hepatic VLDL. PLTP also plays an important role in inflammation and oxidative stress. Accordingly, PLTP has been implicated in the development of atherosclerosis. In this study, we evaluated the association between PLTP activity and lipoprotein metabolism in a Chinese patients cohort with or without coronary heart disease (CHD group n = 407, control group n = 215), the PLTP activity was measured and PLTP genotyping was screened for sequence anomalies by PCR. We found that human plasma PLTP activity was negatively associated with plasma HDL and apoA-I levels, and positively associated with plasma TG, apoB and apoE levels. We also found that PLTP rs2294213 polymorphism was tended to be associated with increased plasma PLTP activity.

PLTP is present in the nucleus, and its nuclear export is CRM1-dependent

Phospholipid transfer protein (PLTP), one of the key lipid transfer proteins in plasma and cerebrospinal fluid, is nearly ubiquitously expressed in cells and tissues. Functions of secreted PLTP have been extensively studied. However, very little is known about potential intracellular PLTP functions. In the current study, we provide evidence for PLTP localization in the nucleus of cells that constitutively express PLTP (human neuroblastoma cells, SK-N-SH; and human cortical neurons, HCN2) and in cells transfected with human PLTP (Chinese hamster ovary and baby hamster kidney cells). Furthermore, we have shown that incubation of these cells with leptomycin B (LMB), a specific inhibitor of nuclear export mediated by chromosome region maintenance 1 (CRM1), leads to intranuclear accumulation of PLTP, suggesting that PLTP nuclear export is CRM1-dependent. We also provide evidence for entry of secreted PLTP into the cell and its translocation to the nucleus, and show that intranuclear PLTP is active in phospholipid transfer. These findings suggest that PLTP is involved in novel intracellular functions.

PLTP activity is a risk factor for subsequent cardiovascular events in CAD patients under statin therapy: the AtheroGene study

Phospholipid transferprotein (PLTP) mediates both net transfer and exchange of phospholipids between different lipoproteins. Although many studies have investigated the role of PLTP in atherogenesis, the role of PLTP in atherosclerotic diseases is unclear. We investigated the association of serum PLTP activity with the incidence of a combined endpoint (myocardial infarction and cardiovascular death) and its relation to other markers of atherosclerosis in 1,085 patients with angiographically documented coronary artery disease (CAD). In the median follow-up of 5.1 years, 156 patients had suffered from the combined endpoint of myocardial infarction or cardiovascular death including 47 of 395 patients who were on statins at baseline. In Kaplan-Meyer analyses serum PLTP activity was not associated with the combined endpoint in all patients. However, in the subgroup of patients receiving statins at baseline, PLTP was shown to be a significant predictor of cardiovascular outcome (P = 0.019), and this also remained stable in univariate (P = 0.027) and multivariate cox regression analyses (P = 0.041) including potential confounders (classical risk factors, HDL cholesterol (HDL-C), and others). We showed in our study that, under statin treatment, high plasma PLTP activity was related to fatal and nonfatal cardiovascular events in CAD patients.

Phospholipid transfer protein deficiency ameliorates diet-induced hypercholesterolemia and inflammation in mice

Phospholipid transfer protein (PLTP) facilitates the transfer of phospholipids from triglyceride-rich lipoproteins into HDL. PLTP has been shown to be an important factor in lipoprotein metabolism and atherogenesis. Here, we report that chronic high-fat, high-cholesterol diet feeding markedly increased plasma cholesterol levels in C57BL/6 mice. PLTP deficiency attenuated diet-induced hypercholesterolemia by dramatically reducing apolipoprotein E-rich lipoproteins (-88%) and, to a lesser extent, LDL (-40%) and HDL (-35%). Increased biliary cholesterol secretion, indicated by increased hepatic ABCG5/ABCG8 gene expression, and decreased intestinal cholesterol absorption may contribute to the lower plasma cholesterol in PLTP-deficient mice. The expression of proinflammatory genes (intercellular adhesion molecule-1 and vascular cell adhesion molecule-1) is reduced in aorta of PLTP knockout mice compared with wild-type mice fed either a chow or a high-cholesterol diet. Furthermore, plasma interleukin-6 levels are significantly lower in PLTP-deficient mice, indicating reduced systemic inflammation. These data suggest that PLTP appears to play a proatherogenic role in diet-induced hyperlipidemic mice.

Macrophage Phospholipid Transfer Protein Contributes Significantly to Total Plasma Phospholipid Transfer Activity and Its Deficiency Leads to Diminished Atherosclerotic Lesion Development

Objective— Systemic phospholipid transfer protein (PLTP) deficiency in mice is associated with a decreased susceptibility to atherosclerosis, whereas overexpression of human PLTP in mice increases atherosclerotic lesion development. PLTP is also expressed by macrophage-derived foam cells in human atherosclerotic lesions, but the exact role of macrophage PLTP in atherosclerosis is unknown.

Methods and Results— To clarify the role of macrophage PLTP in atherogenesis, PLTP was selectively disrupted in hematopoietic cells, including macrophages, by transplantation of bone marrow from PLTP knockout (PLTP −/− ) mice into irradiated low-density lipoprotein receptor knockout mice. Selective deficiency of macrophage PLTP (PLTP −M/−M ) resulted in a 29% ( P <0.01 for difference in lesion area) reduction in aortic root lesion area as compared with mice possessing functional macrophage PLTP (384±36*10 3 μm 2 in the PLTP −M/−M group (n=10), as compared with 539±35*10 3 μm 2 in the PLTP +M/+M group (n=14)) after 9 weeks of Western-type diet feeding. The decreased lesion size in the PLTP −M/−M group coincided with significantly lower serum total cholesterol, free cholesterol, and triglyceride levels in these mice. Furthermore, plasma PLTP activity in the PLTP −M/−M group was 2-fold ( P <0.001) lower than that in the PLTP +M/+M group.

Conclusion— Macrophage PLTP is a significant contributor to plasma PLTP activity and deficiency of PLTP in macrophages leads to lowered atherosclerotic lesion development in low-density lipoprotein receptor knockout mice on Western-type diet.

Predominance of a proinflammatory phenotype in monocyte-derived macrophages from subjects with low plasma HDL-cholesterol

OBJECTIVE

Reduced plasma concentrations of high-density lipoprotein-cholesterol (HDL-C) are a significant risk factor for cardiovascular disease. Mechanisms that regulate HDL-C concentrations represent an important area of investigation.

METHODS AND RESULTS

Comparative transcriptome analyses of monocyte-derived macrophages (MDM) from a large population of low HDL-C subjects and age- and sex-matched controls revealed a cluster of inflammatory genes highly expressed in low HDL-C subjects. The expression levels of peroxisome proliferator activated receptor (PPAR) gamma and several antioxidant metallothionein genes were decreased in MDM from all low HDL-C groups compared with controls, as was the expression of other genes regulated by PPARgamma, including CD36, adipocyte fatty acid binding protein (FABP4), and adipophilin (ADFP). In contrast, PPARdelta expression was increased in MDM from low HDL-C groups. Quantitative RT-PCR corroborated all major findings from the microarray analysis in two separate patient cohorts. Expression of several inflammatory cytokine genes including interleukin 1beta, interleukin 8, and tumor necrosis factor alpha were highly increased in low HDL-C subjects.

CONCLUSIONS

The activated proinflammatory state of monocytes and MDM in low HDL-C subjects constitutes a novel parameter of risk associated with HDL deficiency, related to altered expression of metallothionein genes and the reciprocal regulation of PPARgamma and PPARdelta.

Genetic etiology of isolated low HDL syndrome: incidence and heterogeneity of efflux defects

OBJECTIVE

We have used a multitiered approach to identify genetic and cellular contributors to high-density lipoprotein (HDL) deficiency in 124 human subjects.

METHODS AND RESULTS

We resequenced 4 candidate genes for HDL regulation and identified several functional nonsynonymous mutations including 2 in apolipoprotein A-I (APOA1), 4 in lecithin:cholesterol acyltransferase (LCAT), 1 in phospholipid transfer protein (PLTP), and 7 in the ATP-binding cassette transporter ABCA1, leaving 88% (110/124) of HDL deficient subjects without a genetic diagnosis. Cholesterol efflux assays performed using cholesterol-loaded monocyte-derived macrophages from the 124 low HDL subjects and 48 control subjects revealed that 33% (41/124) of low HDL subjects had low efflux, despite the fact that the majority of these subjects (34/41) were not carriers of dysfunctional ABCA1 alleles. In contrast, only 2% of control subjects presented with low efflux (1/48). In 3 families without ABCA1 mutations, efflux defects were found to cosegregate with low HDL.

CONCLUSIONS

Efflux defects are frequent in low HDL syndromes, but the majority of HDL deficient subjects with cellular cholesterol efflux defects do not harbor ABCA1 mutations, suggesting that novel pathways contribute to this phenotype.

Altered hepatic lipid status and apolipoprotein A-I metabolism in mice lacking phospholipid transfer protein

The effect of PLTP deficiency on hepatic lipid status and apolipoprotein A-I (apoA-I) biosynthesis in PLTP knockout (PLTP-KO) mice was investigated. PLTP-KO mice exhibited a marked reduction in HDL levels, but also increased triglycerides (TG), phospholipids (PL), and cholesterol in very-low-density lipoproteins (VLDL). Both male and female PLTP-KO mice displayed increased hepatic PL and decreased TG, and in the females, increased hepatic cholesterol was also detected. Primary hepatocytes from PLTP-KO mice displayed a different PL molecular species composition to the wild type (WT) controls, with prominent changes being a reduction of long chain fatty acid-containing and an increase of medium chain mono- or di-unsaturated fatty acid containing PL species. Cultured PLTP-KO hepatocytes synthesized and secreted apoA-I in similar quantities as the WT cells. However, the apoA-I secreted by PLTP-KO hepatocytes contained less choline PL, differing also in phosphatidylcholine/sphingomyelin ratio and fatty acyl species composition when compared to apoA-I from WT hepatocytes. Furthermore, the PLTP-KO-derived PL-deficient apoA-I was less stable in the hepatocyte culture medium than that produced by WT cells. These results demonstrate a complex regulatory role of PLTP in serum and liver lipid homeostasis, as well as in the formation of nascent apoA-I-PL complexes from the liver.

Plasma phospholipid transfer protein activity, a determinant of HDL kinetics in vivo

OBJECTIVE

Phospholipid transfer protein (PLTP) is an important regulator in the transport of surface components of triglyceride-rich lipoprotein (TRL) to high density lipoprotein (HDL) during lipolysis and may therefore play an important role in regulating HDL transport. In this study we investigated the relationship of plasma PLTP activity with HDL metabolism in men.

DESIGN AND METHODS

The kinetics of HDL LpA-I and LpA-I:A-II were measured using intravenous administration of [D3]-leucine, gas chromatography-mass spectrometry (GCMS) and a new multicompartmental model for HDL subpopulation kinetics (SAAM II) in 31 men with wide-ranging body mass index (BMI 18-46 kg/m2). Plasma PLTP activity was determined as the transfer of radiolabelled phosphatidylcholine from small unilamellar phosphatidylcholine vesicles to ultracentrifugally isolated HDL.

RESULTS

PLTP activity was inversely associated with LpA-I concentration and production rate (PR) after adjusting for insulin resistance (P < 0.05). No significant associations were observed between plasma PLTP activity and LpA-I fractional catabolic rate (FCR). In multivariate analysis, including homeostasis model assessment score (HOMA), triglyceride, cholesteryl ester transfer protein (CETP) activity and PLTP activity, PLTP activity was the only significant determinant of LpA-I concentration and PR (P = 0.020 and P = 0.016, respectively).

CONCLUSIONS

Plasma PLTP activity may be a significant, independent determinant of LpA-I kinetics in men, and may contribute to the maintenance of the plasma concentration of these lipoprotein particles in setting of hypercatabolism of HDL.

Cathepsin D, a Lysosomal Protease, Regulates ABCA1-mediated Lipid Efflux

To identify genes involved in the regulation of plasma high density lipoprotein (HDL) cholesterol (HDL-C) levels, patients with low HDL-C and age- and sex-matched controls (normal HDL-C) were extensively characterized. Comparative transcriptome analysis was carried out in cholesterol-loaded monocyte-derived macrophages from low HDL subjects segregated into groups with or without cholesterol efflux defects or ABCA1 mutations. Clusters of differentially regulated genes were evident in the low HDL groups as compared with controls. Of particular note, expression of cathepsin D (CTSD), a lysosomal proteinase, was reduced by approximately 50% in monocyte-derived macrophages of low HDL-C subjects, most significantly those with cholesterol efflux defects but without mutations in ABCA1 (p < 0.01). These results were verified by reverse transcription-PCR and replicated in a second cohort. We show here that blocking the activity or expression of CTSD, by pepstatin or CTSD small interfering RNA, respectively, reduced ABCA1 expression and protein abundance in both macrophages and CHO cells and apolipoprotein A-I-mediated lipid efflux by more than 70%. Conversely, expression of CTSD increased both ABCA1 mRNA expression and cellular ABCA1 protein. Consistent with its role in the proteolytic processing of prosaposin, inactivation of CTSD function resulted in the accumulation of glycosphingo-lipid and free cholesterol in late endosomes/lysosomes, a phenotype similar to NPC1 deficiency. Inhibition of CTSD also caused retention of ABCA1 in lysosomal compartments, reducing its trafficking to the plasma membrane. These studies demonstrate a novel and potentially important role for CTSD in intracellular cholesterol trafficking and ABCA1-mediated efflux. Therefore, decreased CTSD expression may contribute to low plasma HDL-C levels.

Alpha-tocopherol modulates phosphatidylserine externalization in erythrocytes: relevance in phospholipid transfer protein-deficient mice

OBJECTIVE

The aim of the present study was to assess the effect of alpha-tocopherol, the main vitamin E isomer on phosphatidylserine (PS) exposure at the surface of circulating erythrocytes, and to determine consequences on erythrocyte properties.

METHODS AND RESULTS

In vitro alpha-tocopherol enrichment of isolated erythrocytes significantly decreased PS externalization as assessed by lower Annexin V-fluorescein isothiocyanate labeling. Plasma phospholipid transfer protein (PLTP) transfers vitamin E, and both alpha- and gamma-tocopherol accumulated in circulating erythrocytes from PLTP-deficient homozygous (PLTP-/-) mice as compared with wild-type mice. In agreement with in vitro studies, vitamin E-enriched erythrocytes from PLTP-/- mice displayed fewer externalized PS molecules than wild-type controls (-64%, P<0.05). The perturbation of phospholipid membrane asymmetry from PLTP-/- erythrocytes was accompanied by impairment of their procoagulant properties, with a 20% increase in clotting time as compared with wild-type controls (P<0.05). Less pronounced, however still significant, changes were observed in alpha-tocopherol content, procoagulant properties, and PS externalization in erythrocytes of PLTP-deficient heterozygotes. Finally, whole blood coagulation and circulating level of D-dimer, which reflects increased thrombus formation in vivo, were significantly decreased in PLTP-/- mice compared with wild-type mice.

CONCLUSIONS

Vitamin E modifies PS externalization in circulating erythrocytes, thus modulating in vivo their PS-dependent procoagulant properties.

Plasma lipid transfer proteins

PURPOSE OF REVIEW

Plasma cholesteryl ester transfer protein and phospholipid transfer protein are involved in lipoprotein metabolism. Conceivably, manipulation of either transfer protein could impact atherosclerosis and other lipid-driven diseases.

RECENT FINDINGS

Cholesteryl ester transfer protein mediates direct HDL cholesteryl ester delivery to the liver cells; adipose tissue-specific overexpression of cholesteryl ester transfer protein in mice reduces the plasma HDL cholesterol concentration and adipocyte size; cholesteryl ester transfer protein TaqIB polymorphism is associated with HDL cholesterol plasma levels and the risk of coronary heart disease. In apolipoprotein B transgenic mice, phospholipid transfer protein deficiency enhances reactive oxygen species-dependent degradation of newly synthesized apolipoprotein B via a post-endoplasmic reticulum process, as well as improving the antiinflammatory properties of HDL in mice. Activity of this transfer protein in cerebrospinal fluid of patients with Alzheimer's disease is profoundly decreased and exogenous phospholipid transfer protein induces apolipoprotein E secretion by primary human astrocytes in vitro.

SUMMARY

Understanding the relationship between lipid transfer proteins and lipoprotein metabolism is expected to be an important frontier in the search for a therapy for atherosclerosis.

Absence of endogenous phospholipid transfer protein impairs ABCA1-dependent efflux of cholesterol from macrophage foam cells

In vitro experiments have demonstrated that exogenous phospholipid transfer protein (PLTP), i.e. purified PLTP added to macrophage cultures, influences ABCA1-mediated cholesterol efflux from macrophages to HDL. To investigate whether PLTP produced by the macrophages (i.e., endogenous PLTP) is also part of this process, we used peritoneal macrophages derived from PLTP-knockout (KO) and wild-type (WT) mice. The macrophages were transformed to foam cells by cholesterol loading, and this resulted in the upregulation of ABCA1. Such macrophage foam cells from PLTP-KO mice released less cholesterol to lipid-free apolipoprotein A-I (apoA-I) and to HDL than did the corresponding WT foam cells. Also, when plasma from either WT or PLTP-KO mice was used as an acceptor, cholesterol efflux from PLTP-KO foam cells was less efficient than that from WT foam cells. After cAMP treatment, which upregulated the expression of ABCA1, cholesterol efflux from PLTP-KO foam cells to apoA-I increased markedly and reached a level similar to that observed in cAMP-treated WT foam cells, restoring the decreased cholesterol efflux associated with PLTP deficiency. These results indicate that endogenous PLTP produced by macrophages contributes to the optimal function of the ABCA1-mediated cholesterol efflux-promoting machinery in these cells. Whether macrophage PLTP acts at the plasma membrane or intracellularly or shuttles between these compartments needs further study.

Plasma phospholipid transfer protein activity is decreased in type 2 diabetes during treatment with atorvastatin: a role for apolipoprotein E?

Plasma phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism. PLTP activity is elevated in patients with diabetes, a condition with strongly elevated risk for coronary heart disease. The aim of this study was to test the hypothesis that statins reduce PLTP activity and to examine the potential role of apolipoprotein E (apoE). PLTP activity and apoE were measured in patients with type 2 diabetes from the DALI (Diabetes Atorvastatin Lipid Intervention) Study, a 30-week randomized double-blind placebo-controlled trial with atorvastatin (10 and 80 mg daily). At baseline, PLTP activity was positively correlated with waist circumference, HbA(1c), glucose, and apoE (all P < 0.05). Atorvastatin treatment resulted in decreased PLTP activity (10 mg atorvastatin: -8.3%, P < 0.05; 80 mg atorvastatin: -12.1%, P < 0.002). Plasma apoE decreased by 28 and 36%, respectively (P < 0.001). The decrease in apoE was strongly related to the decrease in PLTP activity (r = 0.565, P < 0.001). The change in apoE remained the sole determinant of the change in PLTP activity in a multivariate model. The activity of PLTP in type 2 diabetes is decreased by atorvastatin. The association between the decrease in PLTP activity and apoE during statin treatment supports the hypothesis that apoE may prevent PLTP inactivation.

Phosphatidylcholine transfer protein regulates size and hepatic uptake of high-density lipoproteins

Phosphatidylcholine transfer protein (PC-TP) is a steroidogenic acute regulatory-related transfer domain protein that is enriched in liver cytosol and binds phosphatidylcholines with high specificity. In tissue culture systems, PC-TP promotes ATP-binding cassette protein A1-mediated efflux of cholesterol and phosphatidylcholine molecules as nascent pre-beta-high-density lipoprotein (HDL) particles. Here, we explored a role for PC-TP in HDL metabolism in vivo utilizing 8-wk-old male Pctp(-/-) and wild-type littermate C57BL/6J mice that were fed for 7 days with either chow or a high-fat/high-cholesterol diet. In chow-fed mice, neither plasma cholesterol concentrations nor the concentrations and compositions of plasma phospholipids were influenced by PC-TP expression. However, in Pctp(-/-) mice, there was an accumulation of small alpha-migrating HDL particles. This occurred without changes in hepatic expression of ATP-binding cassette protein A1 or in proteins that regulate the intravascular metabolism and clearance of HDL particles. In Pctp(-/-) mice fed the high-fat/high-cholesterol diet, HDL particle sizes were normalized, whereas plasma cholesterol and phospholipid concentrations were increased compared with wild-type mice. In the absence of upregulation of hepatic ATP-binding cassette protein A1, reduced HDL uptake from plasma into livers of Pctp(-/-) mice contributed to higher plasma lipid concentrations. These data indicate that PC-TP is not essential for the enrichment of HDL with phosphatidylcholines but that it does modulate particle size and rates of hepatic clearance.

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.

Active and low-active forms of serum phospholipid transfer protein in a normal Finnish population sample

Human serum phospholipid transfer protein (PLTP) exists as a catalytically active (HA-PLTP) and a low-active (LA-PLTP) form. In this study, the association of PLTP activity and the concentrations of both forms with lipid and carbohydrate parameters were investigated. In a random Finnish population sample, serum PLTP concentration (n=250) was 6.56 +/- 1.45 mg/l, the mean lipoprotein-independent (PLTPexo) phospholipid transfer activity was 6.59 +/- 1.66 micromol/ml/h, and the mean lipoprotein-dependent (PLTPendo) activity was 1.37 +/- 0.29 micromol/ml/h. Of the serum PLTP concentration, approximately 46% was in a catalytically active form. HA-PLTP concentration correlated positively with serum PLTPexo activity (r=0.380, P <0.001), HDL cholesterol (r=0.291, P <0.001), and apolipoprotein A-I (r=0.187, P <0.01). Of the potential regulatory factors for PLTP, apolipoprotein E showed a weak positive correlation with serum PLTPexo (r=0.154, P <0.05) and PLTPendo (r=0.192, P <0.01) activity but not with PLTP concentration. Weak associations were also observed between PLTP parameters and determinants of glucose homeostasis (glucose, insulin, and homeostasis model assessment for insulin resistance). The present data on PLTP activity and concentration reveal novel connections of the two PLTP forms to lipid and carbohydrate metabolism.

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.

Severe hypoalphalipoproteinemia in mice expressing human hepatic lipase deficient in binding to heparan sulfate proteoglycan

Unlike human hepatic lipase (hHL) that is mainly cell surface-anchored via binding to heparan sulfate proteoglycans (HSPG), mouse HL (mHL) has a low affinity to HSPG and thus is largely blood-borne. The reduced HSPG binding of mHL is attributable to the C-terminal amino acids. To determine the functions of HSPG binding of hHL in vivo, we created adenovirus vectors encoding hHL or a chimeric protein (designated hHLmt) in which the C-terminal HSPG-binding sequences were replaced with the corresponding mouse sequences. Injecting hHLmt-expressing virus into C57BL/6J mice (1.8 x 10(10) virus particles/mouse) resulted in a 3-fold increase in pre-heparin HL activity, whereas infection with an identical dose of hHL virus did not change pre-heparin HL activity. In hHLmt-expressing mice, the concentration of total cholesterol and phospholipids was inversely related to the hHL activity in pre-heparin plasma in a dose- and time-dependent manner, and the decrease was mainly attributable to high density lipoproteins (HDL) cholesterol and HDL phospholipids. The expression of hHL exhibited no change in plasma total cholesterol or phospholipid levels as compared with control mice infected with luciferase or injected with saline. The reduced HDL lipids in the hHLmt-expressing mice were accompanied by markedly decreased plasma and hepatic apolipoprotein (apo) A-I. In primary hepatocytes isolated from hHLmt-expressing mice, the concentration of cell-associated and secreted apoA-I was decreased by 2-3-fold as compared with hepatocytes isolated from control mice, whereas the levels of apoB and apoE were unaltered. Infection of primary hepatocytes with hHLmt virus ex vivo also resulted in reduced apoA-I secretion but had no effect on cell-associated apoA-I. These results suggest that expression of HSPG binding-deficient hHL has a profound HDL-lowering effect.

Remodeling of HDL remnants generated by scavenger receptor class B type I

Scavenger receptor class B type I (SR-BI) mediates the selective transfer of cholesteryl ester from HDL to cells. We previously established that SR-BI overexpressed in livers of apolipoprotein A-I-deficient mice processes exogenous human HDL2 to incrementally smaller HDL particles. When mixed with normal mouse plasma either in vivo or ex vivo, SR-BI-generated HDL "remnants" rapidly remodel to form HDL-sized lipoproteins. In this study, we analyzed HDLs throughout the process of HDL remnant formation and investigated the mechanism of conversion to larger particles. Upon interacting with SR-BI, alpha-migrating HDL2 is initially converted to a prealpha-migrating particle that is ultimately processed to a smaller alpha-migrating HDL remnant. SR-BI does not appear to generate prebeta-1 HDL particles. When incubated with isolated lipoprotein fractions, HDL remnants are converted to lipoprotein particles corresponding in size to the particle incubated with the HDL remnant. HDL remnant conversion is not altered in phospholipid transfer protein (PLTP)-deficient mouse plasma or by the addition of purified PLTP. Although LCAT-deficient plasma promoted only partial conversion, this deficiency was attributable to the nature of HDL particles in LCAT-/- mice rather than to a requirement for LCAT in the remodeling process. We conclude that HDL remnants, generated by SR-BI, are converted to larger particles by rapidly reassociating with existing HDL particles in an enzyme-independent manner.

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.

Quantitation of the active and low-active forms of human plasma phospholipid transfer protein by ELISA

Human plasma contains two forms of phospholipid transfer protein (PLTP), one catalytically active [high-activity PLTP (HA-PLTP)] and the other a low-activity (LA-PLTP) form. We present here a PLTP ELISA that allows not only for accurate measurement of PLTP concentration in plasma but also of the distribution of both LA- and HA-PLTP. To achieve similar immunoreactivity of the two PLTP forms, a denaturing sample pretreatment with 0.5% SDS was required. Distribution of LA- and HA-PLTP in plasma was assessed using size-exclusion chromatography, Heparin-Sepharose chromatography, anti-PLTP immunoaffinity chromatography, and dextran sulfate-CaCl2 precipitation. All four methods demonstrated that approximately 60% of plasma PLTP represents LA-PLTP and 40% represents HA-PLTP. According to the modified ELISA, the total serum PLTP concentration in a random Finnish population sample (n = 80) was 5.81 +/- 1.33 mg/l (mean +/- SD) (range, 2.78-10.06 mg/l) and the mean activity was 5.84 +/- 1.39 micromol/ml/h (range, 3.21-11.15 micromol/ml/h). To quantitate both forms of PLTP in sera from this sample, we combined dextran sulfate-CaCl2 precipitation with the modified PLTP ELISA. The HA-PLTP mass (mean, 1.87 +/- 0.85 mg/l) correlated significantly with serum PLTP activity, whereas that of LA-PLTP (mean, 3.94 +/- 1.4 mg/l) showed no correlation with phospholipid transfer activity.

Increased atherosclerotic lesions in apoE mice with plasma phospholipid transfer protein overexpression

OBJECTIVE

Plasma phospholipid transfer protein (PLTP) is involved in the metabolism of HDL and apolipoprotein B (apoB)-containing lipoproteins. Atherosclerosis susceptibility is decreased in mice with PLTP deficiency that is associated with decreased liver production of apoB-containing lipoproteins and increase in their antioxidant. To investigate additionally the effect of PLTP on the development of atherosclerosis, we overexpressed PLTP in mice.

METHODS AND RESULTS

PLTP was overexpressed in apoE knockout mice using an adenovirus-associated virus (AAV)-mediated system. Plasma PLTP activity was 1.3- to 2-fold higher in mice injected with AAV-PLTP than in mice injected with control AAV-GFP, and PLTP levels were sustained during the experiment period (4 months). We show that 2-fold increased PLTP activity results in (1) a decrease in HDL cholesterol, HDL phospholipid, and apoAI levels; (2) a decrease in vitamin E contents in total plasma and in individual lipoprotein fractions; (3) an increase in lipoprotein oxidizability as assessed by copper-induced formation of conjugated dienes; (4) an increase in autoantibodies against oxidized apoB-containing particles; and (5) an increase in atherosclerosis lesions in proximal aorta.

CONCLUSIONS

These observations indicate that elevated plasma PLTP levels constitute a novel, long-term risk factor for atherosclerosis.