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

Analysis of the heterogeneity and complexity of murine extraorbital lacrimal gland via single-cell RNA sequencing

PURPOSE

The lacrimal gland is essential for maintaining ocular surface health and avoiding external damage by secreting an aqueous layer of the tear film. However, a healthy lacrimal gland's inventory of cell types and heterogeneity remains understudied.

METHODS

Here, 10X Genome-based single-cell RNA sequencing was used to generate an unbiased classification of cellular diversity in the extraorbital lacrimal gland (ELG) of C57BL/6J mice. From 43,850 high-quality cells, we produced an atlas of cell heterogeneity and defined cell types using classic marker genes. The possible functions of these cells were analyzed through bioinformatics analysis. Additionally, the CellChat was employed for a preliminary analysis of the cell-cell communication network in the ELG.

RESULTS

Over 37 subclasses of cells were identified, including seven types of glandular epithelial cells, three types of fibroblasts, ten types of myeloid-derived immune cells, at least eleven types of lymphoid-derived immune cells, and five types of vascular-associated cell subsets. The cell-cell communication network analysis revealed that fibroblasts and immune cells play a pivotal role in the dense intercellular communication network within the mouse ELG.

CONCLUSIONS

This study provides a comprehensive transcriptome atlas and related database of the mouse ELG.

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.

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.

Acyl-CoA:Diacylglycerol Acyltransferase 1 Expression Level in the Hematopoietic Compartment Impacts Inflammation in the Vascular Plaques of Atherosclerotic Mice

The final step of triacylglycerol synthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferases (DGATs). We have previously shown that ApoE^-/-Dgat1^-/- mice are protected from developing atherosclerosis in association with reduced foam cell formation. However, the role of DGAT1, specifically in myeloid and other hematopoietic cell types, in determining this protective phenotype is unknown. To address this question, we reconstituted the bone marrow of irradiated Ldlr^–/–mice with that from wild-type (WT→ Ldlr^–/–) and Dgat1^–/–(Dgat1^–/–→ Ldlr^–/–) donor mice. We noted that DGAT1 in the hematopoietic compartment exerts a sex-specific effect on systemic cholesterol homeostasis. However, both male and female Dgat1^–/–→ Ldlr^–/–mice had higher circulating neutrophil and lower lymphocyte counts than control mice, suggestive of a classical inflammatory phenotype. Moreover, specifically examining the aortae of these mice revealed that Dgat1^–/–→ Ldlr^–/–mice have atherosclerotic plaques with increased macrophage content. This increase was coupled to a reduced plaque collagen content, leading to a reduced collagen-to-macrophage ratio. Together, these findings point to a difference in the inflammatory contribution to plaque composition between Dgat1^–/–→ Ldlr^–/–and control mice. By contrast, DGAT1 deficiency did not affect the transcriptional responses of cultured macrophages to lipoprotein treatment in vitro, suggesting that the alterations seen in the plaques of Dgat1^–/–→ Ldlr^–/–mice in vivo do not reflect a cell intrinsic effect of DGAT1 in macrophages. We conclude that although DGAT1 in the hematopoietic compartment does not impact the overall lipid content of atherosclerotic plaques, it exerts reciprocal effects on inflammation and fibrosis, two processes that control plaque vulnerability.

The binding capability of plasma phospholipid transfer protein, but not HDL pool size, is critical to repress LPS induced inflammation

Phospholipid transfer protein (PLTP) participates in high density lipoprotein (HDL) metabolism. Increased plasma PLTP activity was observed in lipopolysaccharide (LPS) triggered acute inflammatory diseases. This study aimed to determine the exact role of PLTP in LPS induced inflammation. HDL pool size was shrunk both in PLTP deficient mice (PLTP−/−) and PLTP transgenic mice (PLTP-Tg). PLTP displayed a strong protective effect on lethal endotoxemia in mice survival study. Furthermore, after LPS stimulation, the expression of pro-inflammatory cytokines were increased in bone marrow derived macrophage (BMDM) from PLTP−/−, while decreased in BMDM from PLTP-Tg compared with BMDM from wild-type mice (WT). Moreover, LPS induced nuclear factor kappa-B (NFκB) activation was enhanced in PLTP−/− BMDM or PLTP knockdown RAW264.7. Conversely, PLTP overexpression countered the NFκB activation in LPS challenged BMDM. Additionally, the activation of toll like receptor 4 (TLR4) induced by LPS showed no alteration in PLTP−/− BMDM. Finally, PLTP could bind to LPS, attenuate the pro-inflammatory effects of LPS, and improve the cell viability in vitro. To sum up, these findings elucidated that PLTP repressed LPS induced inflammation due to extracellular LPS binding capability, and the protective effects were not related to HDL pool size in mice.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

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.

Prolactin receptor antagonism uncouples lipids from atherosclerosis susceptibility

The pituitary-derived hormone prolactin has been suggested to stimulate the development of atherosclerosis and cardiovascular disease through its effects on metabolism and inflammation. In this study, we aimed to challenge the hypothesis that inhibition of prolactin function may beneficially affect atherosclerosis burden. Hereto, atherosclerosis-susceptible LDL receptor (Ldlr) knockout mice were transplanted with bone marrow from transgenic mice expressing the pure prolactin receptor antagonist Del1-9-G129R-hPRL or their non-transgenic littermates as control. Recipient mice expressing Del1-9-G129R-hPRL exhibited a decrease in plasma cholesterol levels (-29%; P<0.05) upon feeding a Western-type diet (WTD), which could be attributed to a marked decrease (-47%; P<0.01) in the amount of cholesterol esters associated with pro-atherogenic lipoproteins VLDL/LDL. By contrast, Del1-9-G129R-hPRL-expressing mice did not display any change in the susceptibility for atherosclerosis after 12 weeks of WTD feeding. Both the absolute atherosclerotic lesion size (223 ± 33 × 10(3) μm(2) for Del1-9-G129R-hPRL vs 259 ± 32 × 10(3) μm(2) for controls) and the lesional macrophage and collagen contents were not different between the two groups of bone marrow recipients. Importantly, Del1-9-G129R-hPRL exposure increased levels of circulating neutrophils (+91%; P<0.05), lymphocytes (+55%; P<0.05), and monocytes (+43%; P<0.05), resulting in a 49% higher (P<0.01) total blood leukocyte count. In conclusion, we have shown that prolactin receptor signaling inhibition uncouples the plasma atherogenic index from atherosclerosis susceptibility in Ldlr knockout mice. Despite an associated decrease in VLDL/LDL cholesterol levels, application of the prolactin receptor antagonist Del1-9-G129R-hPRL does not alter the susceptibility for initial development of atherosclerotic lesions probably due to the parallel increase in circulating leukocyte concentrations.

The macrophage LBP gene is an LXR target that promotes macrophage survival and atherosclerosis

The liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate sterol metabolism and inflammation. We sought to identify previously unknown genes regulated by LXRs in macrophages and to determine their contribution to atherogenesis. Here we characterize a novel LXR target gene, the lipopolysaccharide binding protein (LBP) gene. Surprisingly, the ability of LXRs to control LBP expression is cell-type specific, occurring in macrophages but not liver. Treatment of macrophages with oxysterols or loading with modified LDL induces LBP in an LXR-dependent manner, suggesting a potential role for LBP in the cellular response to cholesterol overload. To investigate this further, we performed bone marrow transplant studies. After 18 weeks of Western diet feeding, atherosclerotic lesion burden was assessed revealing markedly smaller lesions in the LBP(-/-) recipients. Furthermore, loss of bone marrow LBP expression increased apoptosis in atherosclerotic lesions as determined by terminal deoxynucleotidyl transferase dUTP nick end labeling staining. Supporting in vitro studies with isolated macrophages showed that LBP expression does not affect cholesterol efflux but promotes the survival of macrophages in the setting of cholesterol loading. The LBP gene is a macrophage-specific LXR target that promotes foam cell survival and atherogenesis.

Development of abdominal aortic aneurysm is decreased in mice with plasma phospholipid transfer protein deficiency

Plasma phospholipid transfer protein (PLTP) increases the circulating levels of proatherogenic lipoproteins, accelerates blood coagulation, and modulates inflammation. The role of PLTP in the development of abdominal aortic aneurysm (AAA) was investigated by using either a combination of mechanical and elastase injury at one site of mouse aorta (elastase model) or continuous infusion of angiotensin II in hyperlipidemic ApoE-knockout mice (Ang II model). With the elastase model, complete PLTP deficiency was associated with a significantly lower incidence and a lesser degree of AAA expansion. With the Ang II model, findings were consistent with those in the elastase model, with a lower severity grade in PLTP-deficient mice, an intermediate phenotype in PLTP-deficient heterozygotes, and a blunted effect of the PLTP-deficient trait when restricted to bone marrow-derived immune cells. The protective effect of whole-body PLTP deficiency in AAA was illustrated further by a lesser degree of adventitia expansion, reduced elastin degradation, fewer recruited macrophages, and less smooth muscle cell depletion in PLTP-deficient than in wild-type mice, as evident from comparative microscopic analysis of aorta sections. Finally, cumulative evidence supports the association of PLTP deficiency with reduced expression and activity levels of matrix metalloproteinases, known to degrade elastin and collagen. We conclude that PLTP can play a significant role in the pathophysiology of AAA.

Elevated expression of PLTP is atherogenic in apolipoprotein E deficient mice

OBJECTIVE

Plasma phospholipid transfer protein (PLTP) plays a key role in lipoprotein metabolism. Its exact function in the development of atherosclerosis is still under debate however. We studied the effect of elevated PLTP expression in one of the most commonly used models of atherosclerosis, the ApoE deficient mouse.

METHODS

Experiment 1: Plasma PLTP activity, total cholesterol, HDL cholesterol and atherosclerosis development was measured in ApoE deficient mice with or without elevated expression of PLTP. Experiment 2: The same parameters were measured in ApoE deficient mice after bone marrow transplantation from wild type mice or mice with elevated PLTP expression. Experiment 3: Similar to experiment 2, but using donor mice with an ApoE deficient background.

RESULTS

Experiment 1: ApoE deficient mice have more than two times more atherosclerosis when overexpressing PLTP and a strongly decreased plasma level of HDL. Experiment 2: Bone marrow transplantation with ApoE proficient cells results in a strong reduction of plasma cholesterol in ApoE deficient acceptor mice. Still, elevated PLTP in bone marrow derived cells evoke a reduction of HDL cholesterol and increased atherosclerosis. Experiment 3: Bone marrow transplantation with ApoE deficient cells results in much higher cholesterol levels, but here too HDL cholesterol levels are reduced and atherosclerosis increased.

CONCLUSION

In all the models with ApoE deficiency, elevated PLTP expression causes higher levels of diet-induced atherosclerosis coinciding with decreased plasma levels of HDL cholesterol.

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

It has been reported that phospholipid transfer protein (PLTP) is an independent risk factor for human coronary artery disease. In mouse models, it has been demonstrated that PLTP overexpression induces atherosclerosis, while its deficiency reduces it. PLTP is considered a promising target for pharmacological intervention to treat atherosclerosis. However, we must still answer a number of questions before its pharmaceutical potential can be fully explored. In this review, we summarized the recent progresses made in the PLTP research field and focused on its effect on apoB-containing- triglyceride-rich particle and HDL metabolism.

Clinical significance of serum lipids in idiopathic pulmonary alveolar proteinosis

BACKGROUND

It is well known that pulmonary alveolar proteinosis(PAP) is characterised by accumulation of surfactant lipids and proteins within airspaces. However, few previous data describe the serum lipid levels associated with PAP.

MATERIALS AND METHODS

We retrospectively reviewed 25 patients with idiopathic PAP(iPAP). The serum lipid levels of patients with idiopathic PAP were compared with those of the healthy volunteers. In patients and healthy subjects, the LDL-C/HDL-C ratios were 2.94 ± 1.21 and 1.60 ± 0.70, respectively (p < 0.001), HDL-C were 1.11 ± 0.27 and 1.71 ± 0.71 respectively (p < 0.001). The values of LDL-C correlated significantly with those of PaO2 and PA-aO2 (r = -0.685, p = 0.003, and r = 0.688, p = 0.003, respectively). The values of LDL-C/HDL-C ratios also correlated with PaO2 levels and PA-aO2 levels (r = -0.698, p = 0.003, and r = 0.653, p = 0.006, respectively). 11 and 13 patients experienced respectively a decline in TC and LDL-C levels following whole lung lavage(WLL), the median decline was 0.71 mmol/L(p < 0.009) and 0.47 mmol/L(p < 0.003), respectively.

CONCLUSIONS

the serum lipid levels, especially the levels of LDL-C and LDL-C/HDL-C, may reflect the severity of the disease in PAP patients, and predict the therapeutic effect of WLL.

Plasma PLTP (phospholipid-transfer protein): an emerging role in 'reverse lipopolysaccharide transport' and innate immunity

Plasma PLTP (phospholipid-transfer protein) is a member of the lipid transfer/LBP [LPS (lipopolysaccharide)-binding protein] family, which constitutes a superfamily of genes together with the short and long PLUNC (palate, lung and nasal epithelium clone) proteins. Although PLTP was studied initially for its involvement in the metabolism of HDL (high-density lipoproteins) and reverse cholesterol transport (i.e. the metabolic pathway through which cholesterol excess can be transported from peripheral tissues back to the liver for excretion in the bile), it displays a number of additional biological properties. In particular, PLTP can modulate the lipoprotein association and metabolism of LPS that are major components of Gram-negative bacteria. The delayed association of LPS with lipoproteins in PLTP-deficient mice results in a prolonged residence time, in a higher toxicity of LPS aggregates and in a significant increase in LPS-induced mortality as compared with wild-type mice. It suggests that PLTP may play a pivotal role in inflammation and innate immunity through its ability to accelerate the 'reverse LPS transport' pathway.

Impact of lung transplantation on serum lipids in COPD

BACKGROUND

Severe chronic obstructive pulmonary disease is associated with high HDL cholesterol (HDL-C). We sought to examine the effect of lung transplantation on lipid profiles in patients with COPD.

METHODS

We analyzed 101 lung transplant recipients in a retrospective cohort of patients from two centers in whom lipid values were available both before as well as after transplantation. Sixty-one subjects were transplanted for severe COPD (93% GOLD stage 4).

RESULTS

Eighty-nine percent of subjects with COPD exhibited a decline in HDL-C. Median decline for the COPD cohort was 25 mg/dL (IQR 12-38 mg/dL, p < 0.0001). Non-COPD subjects exhibited no significant changes in HDL-C. Other lipid changes in the COPD cohort included a rise in triglycerides of 70 mg/dL (IQR 35 to 140, p < 0.0001). Decreases in HDL-C levels were independent from the rise in triglyceride levels. Neither LDL-C nor non-HDL-C demonstrated significant changes. Subjects with greater increases in prednisone exposure post-transplant exhibited lesser declines in HDL-C. Compared with tacrolimus, cyclosporine had no effect on observed changes in HDL-C or triglycerides, but was associated with a greater median rise in LDL-C.

CONCLUSIONS

In patients with COPD, lung transplantation results in reductions in the serum levels of HDL-C. These changes are not observed in patients undergoing lung transplantation for diagnoses other than COPD.

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.

PLTP regulates STAT3 and NFκB in differentiated THP1 cells and human monocyte-derived macrophages

Phospholipid transfer protein (PLTP) plays an important role in regulation of inflammation. Previously published studies have shown that PLTP binds, transfers and neutralizes bacterial lipopolysaccharides. In the current study we tested the hypothesis that PLTP can also regulate anti-inflammatory pathways in macrophages. Incubation of macrophage-like differentiated THP1 cells and human monocyte-derived macrophages with wild-type PLTP in the presence or absence of tumor necrosis factor alpha (TNFα) or interferon gamma (IFNγ) significantly increased nuclear levels of active signal transducer and activator of transcription 3, pSTAT3(Tyr705) (p<0.01). Similar results were obtained in the presence of a PLTP mutant without lipid transfer activity (PLTP(M159E)), suggesting that PLTP-mediated lipid transfer is not required for activation of the STAT3 pathway. Inhibition of ABCA1 by chemical inhibitor, glyburide, as well as ABCA1 RNA inhibition, reversed the observed PLTP-mediated activation of STAT3. In addition, PLTP reduced nuclear levels of active nuclear factor kappa-B (NFκB) p65 and secretion of pro-inflammatory cytokines in conditioned media of differentiated THP1 cells and human monocyte-derived macrophages. Our data suggest that PLTP has anti-inflammatory capabilities in macrophages.

Role of plasma phospholipid transfer protein in lipid and lipoprotein metabolism

The understanding of the physiological and pathophysiological role of PLTP has greatly increased since the discovery of PLTP more than a quarter of century ago. A comprehensive review of PLTP is presented on the following topics: PLTP gene organization and structure; PLTP transfer properties; different forms of PLTP; characteristics of plasma PLTP complexes; relationship of plasma PLTP activity, mass and specific activity with lipoprotein and metabolic factors; role of PLTP in lipoprotein metabolism; PLTP and reverse cholesterol transport; insights from studies of PLTP variants; insights of PLTP from animal studies; PLTP and atherosclerosis; PLTP and signal transduction; PLTP in the brain; and PLTP in human disease. PLTP's central role in lipoprotein metabolism and lipid transport in the vascular compartment has been firmly established. However, more studies are needed to further delineate PLTP's functions in specific tissues, such as the lung, brain and adipose tissue. Furthermore, the specific role that PLTP plays in human diseases, such as atherosclerosis, cancer, or neurodegenerative disease, remains to be clarified. Exciting directions for future research include evaluation of PLTP's physiological relevance in intracellular lipid metabolism and signal transduction, which undoubtedly will advance our knowledge of PLTP functions in health and disease. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Vitamin E transport, membrane incorporation and cell metabolism: Is alpha-tocopherol in lipid rafts an oar in the lifeboat?

Vitamin E is composed of closely related compounds, including tocopherols and tocotrienols. Studies of the last decade provide strong support for a specific role of alpha-tocopherol in cell signalling and the regulation of gene expression. It produces significant effects on inflammation, cell proliferation and apoptosis that are not shared by other vitamin E isomers with similar antioxidant properties. The different behaviours of vitamin E isomers might relate, at least in part, to the specific effects they exert at the plasma membrane. alpha-Tocopherol is not randomly distributed throughout the phospholipid bilayer of biological membranes, and as compared with other isomers, it shows a propensity to associate with lipid rafts. Distinct aspects of vitamin E transport and metabolism is discussed with emphasis on the interaction between alpha-tocopherol and lipid rafts and the consequences of these interactions on cell metabolism.

Phospholipid transfer protein in human plasma associates with proteins linked to immunity and inflammation

Phospholipid transfer protein (PLTP), which associates with apolipoprotein A-I (the major HDL protein), plays a key role in lipoprotein remodeling. Because its level in plasma increases during acute inflammation, it may also play previously unsuspected roles in the innate immune system. To gain further insight into its potential physiological functions, we isolated complexes containing PLTP from plasma by immunoaffinity chromatography and determined their composition. Shotgun proteomics revealed that only 6 of the 24 proteins detected in the complexes were apolipoproteins. The most abundant proteins were clusterin (apoJ), PLTP itself, coagulation factors, complement factors, and apoA-I. Remarkably, 20 of the 24 proteins had known protein-protein interactions. Biochemical studies confirmed two previously established interactions and identified five new ones between PLTP and proteins. Moreover, clusterin, apoA-I, and apoE preserved the lipid-transfer activity of recombinant PLTP in the absence of lipid, indicating that these interactions may have functional significance. Unexpectedly, lipids accounted for only 3% of the mass of the PLTP complexes. Collectively, our observations indicate that PLTP in human plasma resides on lipid-poor complexes dominated by clusterin and proteins implicated in host defense and inflammation. They further suggest that protein-protein interactions drive the formation of PLTP complexes in plasma.

Human apoA-I increases macrophage foam cell derived PLTP activity without affecting the PLTP mass

BACKGROUND

phospholipid transfer protein (PLTP) plays important roles in lipoprotein metabolism and atherosclerosis and is expressed by macrophages and macrophage foam cells (MFCs). The aim of the present study was to determine whether the major protein from HDL, apoA-I, affects PLTP derived from MFCs.

RESULTS

as cell model we used human THP-1 monocytes incubated with acetylated LDL, to generate MFC. The addition of apoA-I to the cell media increased apoE secretion from the cells, in a concentration dependent fashion, without affecting cellular apoE levels. In contrast, apoA-I had no effect on PLTP synthesis and secretion, but strongly induced the PLTP activity in the media. ApoA-I also increased phospholipid transfer activity of PLTP isolated from human plasma. This effect was dependent on apoA-I concentration but independent on apoA-I lipidation status. ApoE, ApoA-II and apoA-IV, but not immunoglobulins or bovine serum albumin, also increased PLTP activity. We also report that apoA-I protects PLTP from heat inactivation.

CONCLUSION

apoA-I enhances the phospholipid transfer activity of PLTP secreted from macrophage foam cells without affecting the PLTP mass.

Mast cells promote atherosclerosis by inducing both an atherogenic lipid profile and vascular inflammation

Accumulating in vitro and in vivo studies have proposed a role for mast cells in the pathogenesis of atherosclerosis. Here, we studied the role of mast cells in lipoprotein metabolism, a key element in the atherosclerotic disease. Male mice deficient in low-density lipoprotein receptors and mast cells on a Western diet for 26 weeks had significantly less atherosclerotic changes both in aortic sinus (55%, P = 0.0009) and in aorta (31%, P = 0.049), as compared to mast cell-competent littermates. Mast cell-deficient female mice had significantly less atherosclerotic changes in aortic sinus (43%, P = 0.011). Furthermore, we found a significant positive correlation between the extent of atherosclerosis and the number of adventitial/perivascular mast cells in aortic sinus of mast cell-competent mice (r = 0.615, P = 0.015). Serum cholesterol and triglyceride levels were significantly lower in both male (63%, P = 0.0005 and 57%, P = 0.004) and female (73%, P = 0.00009 and 54%, P = 0.007) mast cell-deficient mice, with a concomitant decrease in atherogenic apoB-containing particles and serum prebeta-high-density lipoprotein and phospholipid transfer protein activity in both male (69% and 24%) and female (74% and 54%) mast cell-deficient mice. Serum soluble intercellular adhesion molecule was decreased in both male (32%, P = 0.004) and female (28%, P = 0.003) mast cell-deficient mice, whereas serum amyloid A was similar between mast cell-deficient and competent mice. In conclusion, mast cells participate in the pathogenesis of atherosclerosis in ldlr(-/-) mice by inducing both an atherogenic lipid profile and vascular inflammation.

Increased biglycan in aortic valve stenosis leads to the overexpression of phospholipid transfer protein via Toll-like receptor 2

Aortic stenosis (AS) is the most common valvular heart disease, and it is suspected that atherosclerotic mechanisms are involved in the development of this disorder. Therefore, the retention of lipids within the aortic valve may play a role in the pathobiology of AS. In this study, a gene expression microarray experiment was conducted on human aortic valves with and without AS. The expression levels of transcripts encoding proteoglycans and enzymes involved in lipid retention were compared between the two groups. The microarray results were subsequently replicated in a cohort of 87 AS valves and 36 control valves. In addition, the interaction between proteoglycan and lipid-modifying enzyme was documented in isolated valve interstitial cells (VICs). The microarray results indicated that only biglycan (BGN) and phospholipid transfer protein (PLTP) were overexpressed in the AS valves. These results were then confirmed by quantitative PCR. The immunohistochemical analysis revealed a colocalization of BGN, PLTP, and Toll-like receptor-2 (TLR 2) in AS valves. In vitro, we showed that BGN induces the production of PLTP in VICs via the stimulation of TLR 2. Thus, increased accumulation of BGN in AS valves contributes to the production of PLTP via TLR 2. These results suggest that intricate links between valve matrix proteins, inflammation, and lipid retention are involved in the pathobiology of AS.

Genetic and nongenetic sources of variation in phospholipid transfer protein activity

Phospholipid transfer protein (PLTP) belongs to the lipid transfer/lipopolysaccharide-binding protein gene family. Expression of PLTP has been implicated in the development of atherosclerosis. We evaluated the effects of PLTP region tagging single nucleotide polymorphisms (SNPs) on the prediction of both carotid artery disease (CAAD) and PLTP activity. CAAD effects were evaluated in 442 Caucasian male subjects with severe CAAD and 497 vascular disease-free controls. SNP prediction of PLTP transfer activity was evaluated in both a subsample of 87 subjects enriched for an allele of interest and in a confirmation sample of 210 Caucasian males and females. Hemoglobin A1c or insulin level predicted 11-14% of age- and sex-adjusted PLTP activity. PLTP SNPs that predicted approximately 11-30% of adjusted PLTP activity variance were identified in the two cohorts. For rs6065904, the allele that was associated with CAAD was also associated with elevated PLTP activity in both cohorts. SNPs associated with PLTP activity also predicted variation in LDL-cholesterol and LDL-B level only in the replication cohort. These results demonstrate that PLTP activity is strongly influenced by PLTP region polymorphisms and metabolic factors.

Liver X receptor-mediated induction of cholesteryl ester transfer protein expression is selectively impaired in inflammatory macrophages

OBJECTIVE

Cholesteryl ester transfer protein (CETP) is a target gene for the liver X receptor (LXR). The aim of this study was to further explore this regulation in the monocyte-macrophage lineage and its modulation by lipid loading and inflammation, which are key steps in the process of atherogenesis.

METHODS AND RESULTS

Exposure of bone marrow-derived macrophages from human CETP transgenic mice to the T0901317 LXR agonist increased CETP, PLTP, and ABCA1 mRNA levels. T0901317 also markedly increased CETP mRNA levels and CETP production in human differentiated macrophages, whereas it had no effect on CETP expression in human peripheral blood monocytes. In inflammatory mouse and human macrophages, LXR-mediated CETP gene upregulation was inhibited, even though ABCA1, ABCG1, and SREBP1c inductions were maintained. The inhibition of CETP gene response to LXR agonists in inflammatory cells was independent of lipid loading (ie, oxidized LDL increased CETP production in noninflammatory macrophages with a synergistic effect of synthetic LXR agonists).

CONCLUSIONS

LXR-mediated induction of human CETP expression is switched on during monocyte-to-macrophage differentiation, is magnified by lipid loading, and is selectively lost in inflammatory macrophages, which suggests that inflammatory cells may not increase the circulating CETP pool on LXR agonist treatment.

Lipid transfer proteins: past, present and perspectives

Lipid transfer proteins (PLTP and CETP) play roles in atherogenesis by modifying the arterial intima cholesterol content via altering the concentration and function of plasma lipoproteins and influencing inflammation. In this regard, endotoxins impair the reverse cholesterol transport (RCT) system in an endotoxemic rodent model, supporting a pro-inflammatory role of HDL reported in chronic diseases where atherosclerosis is premature. High PLTP activity related to atherosclerosis in some clinical studies, but the mechanisms involved could not be ascertained. In experimental animals the relation of elevated plasma PLTP concentration with atherosclerosis was confounded by HDL-C lowering and by unfavorable effects on several inflammatory markers. Coincidently, PLTP also increases in human experimental endotoxemia and in clinical sepsis. Human population investigations seem to favor low CETP as atheroprotective; this is supported by animal models where overexpression of huCETP is atherogenic, most likely due to increased concentration of apoB-lipoprotein-cholesterol. Thus, in spite of CETP facilitating the HDL-C-mediated RCT, the reduction of apoB-LP-cholesterol concentration is the probable antiatherogenic mechanism of CETP inhibition. On the other hand, experimental huCETP expression protects mice from the harmful effects of a bacterial polysaccharide infusion and the mortality rate of severely ill patients correlates with reduction of the plasma CETP concentration. Thus, the roles played by PLTP and CETP on atherosclerosis and acute inflammation seem contradictory. Therefore, the biological roles of PLTP and CETP must be carefully monitored when investigating drugs that inhibit their activity in the prevention of atherosclerosis.

Plasma phospholipid transfer protein (PLTP): review of an emerging cardiometabolic risk factor

Plasma phospholipid transfer protein (PLTP) is a lipid transfer glycoprotein that binds to and transfers a number of amphipathic compounds. In earlier studies, the attention of the scientific community focused on the positive role of PLTP in high-density lipoprotein (HDL) metabolism. However, this potentially anti-atherogenic role of PLTP has been challenged recently by another picture: PLTP arose as a pro-atherogenic factor through its ability to increase the production of apolipoprotein B-containing lipoproteins, to decrease their antioxidative protection and to trigger inflammation. In humans, PLTP has mostly been studied in patients with cardiometabolic disorders. Both PLTP and related cholesteryl ester transfer protein (CETP) are secreted proteins, and adipose tissue is an important contributor to the systemic pools of these two proteins. Coincidently, high levels of PLTP and CETP have been found in the plasma of obese patients. PLTP activity and mass have been reported to be abnormally elevated in type 2 diabetes mellitus (T2DM) and insulin-resistant states, and this elevation is frequently associated with hypertriglyceridemia and obesity. This review article presents the state of knowledge on the implication of PLTP in lipoprotein metabolism, on its atherogenic potential, and the complexity of its implication in obesity, insulin resistance and T2DM.

SAA and PLTP activity in plasma of periodontal patients before and after full-mouth tooth extraction

OBJECTIVE

To assess whether treatment of advanced periodontal disease affects plasma levels of serum amyloid A (SAA) and phospholipid transfer protein (PLTP) activity.

DESIGN

We measured the levels of SAA and PLTP activity in plasma of 66 patients with advanced periodontal disease before and after treatment by full-mouth tooth extraction (FME).

RESULTS

At baseline, median SAA levels in our study population were within the normal range (2.7 microg ml(-1)) but SAA was elevated (>5 microg ml(-1)) in 18% of periodontitis patients. Three months after FME, SAA levels were significantly reduced (P = 0.04). SAA did not correlate with any of the periodontal disease parameters. PLTP activity was elevated in patients with periodontitis, compared to the PLTP activity reference group (age-matched systemically healthy adults, n = 29; 18 micromol ml(-1) h(-1)vs 13 micromol ml(-1) h(-1), respectively, P = 0.002). PLTP activity inversely correlated with average periodontal pocket depth (PPD) per tooth (r(s) = -0.372; P = 0.002). Three months after FME, median PLTP activity did not change significantly.

CONCLUSIONS

Full-mouth tooth extraction significantly reduces SAA, a marker of inflammation, while it does not affect plasma PLTP activity. However, the inverse correlation between PLTP activity and average PPD suggests that increased PLTP activity may limit periodontal tissue damage.

The role of plasma lipid transfer proteins in lipoprotein metabolism and atherogenesis

The plasma lipid transfer proteins promote the exchange of neutral lipids and phospholipids between the plasma lipoproteins. Cholesteryl ester transfer protein (CETP) facilitates the removal of cholesteryl esters from HDL and thus reduces HDL levels, while phospholipid transfer protein (PLTP) promotes the transfer of phospholipids from triglyceride-rich lipoproteins into HDL and increases HDL levels. Studies in transgenic mouse models and in humans with rare genetic deficiencies (CETP) or common genetic variants (CETP and PLTP) highlight the central role of these molecules in regulating HDL levels. Human CETP deficiency is associated with dramatic elevations of HDL cholesterol and apolipoprotein A-I levels, while PLTP variants with increased expression are associated with higher HDL levels. A recent meta-analysis suggests that common CETP alleles causing reduced CETP and increased HDL levels are associated with reduced coronary heart disease. The failure of a clinical trial with the CETP inhibitor torcetrapib may have been related in part to off-target toxicity. Ongoing phase 3 clinical trials with other CETP inhibitors may help to clarify if this strategy can ultimately be successful in the treatment of atherosclerosis.

Elevation of systemic PLTP, but not macrophage-PLTP, impairs macrophage reverse cholesterol transport in transgenic mice

Phospholipid transfer protein (PLTP) is a multifunctional protein synthesized by various cell types and secreted into the plasma. Plasma PLTP is able to transfer phospholipids between lipoproteins and modulate HDL particles. Mice with overexpression of human PLTP have an increased ability to generate pre beta-HDL, reduced total HDL levels and an increased susceptibility to atherosclerosis. As the macrophage is a key component of the atherosclerotic lesion and an important site of PLTP expression, we investigated the role of systemic and peripheral PLTP in macrophage cholesterol efflux and reverse cholesterol transport (RCT) in vivo. We used an assay in which (3)H-labelled cholesterol-loaded macrophages were injected intraperitoneally into recipient mice, and radioactivity was quantified in plasma, liver and faeces. Firstly, wild type macrophages were injected into wild type, PLTP transgenic (PLTPtg) and apoAI transgenic (apoAItg) mice. While plasma (3)H-tracer levels in apoAItg mice were increased compared with wild type mice, they were reduced in PLTPtg mice. Moreover, overexpression of PLTP significantly decreased faecal (3)H-tracer levels compared with wild type and apoAItg mice. Secondly, wild type mice were injected with peritoneal macrophages derived from PLTPtg or wild type mice. No significant difference in the amount of (3)H-tracer in plasma, liver or faeces was found between the two groups of mice. Our findings demonstrate that macrophage cholesterol efflux and RCT to faeces is impaired in PLTP transgenic mice, and that elevation of macrophage-PLTP does not affect RCT, indicating that higher systemic PLTP levels may promote atherosclerosis development by decreasing the rate of macrophage RCT.

Leukocyte-derived hepatic lipase increases HDL and decreases en face aortic atherosclerosis in LDLr-/- mice expressing CETP

In addition to hepatic expression, cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) are expressed by human macrophages. The combined actions of these proteins have profound effects on HDL structure and function. It is not known how these HDL changes influence atherosclerosis. To elucidate the role of leukocyte-derived HL on atherosclerosis in a background of CETP expression, we studied low density lipoprotein receptor-deficient mice expressing human CETP (CETPtgLDLr -/-) with a leukocyte-derived HL deficiency (HL -/- BM). HL(-/-) bone marrow (BM), CETPtgLDLr(-/-) mice were generated via bone marrow transplantation. Wild-type bone marrow was transplanted into CETPtgLDLr(-/-) mice to generate HL +/+ BM, CETPtgLDLr(-/-) controls. The chimeras were fed a high-fat, high-cholesterol diet for 14 weeks to promote atherosclerosis. In female HL(-/-) BM, CETPtgLDLr(-/-) mice plasma HDL-cholesterol concentration during high-fat feeding was decreased 27% when compared with HL +/+ BM, CETPtgLDLr(-/-) mice (P < 0.05), and this was associated with a 96% increase in en face aortic atherosclerosis (P < 0.05). In male CETPtgLDLr(-/-) mice, leukocyte-derived HL deficiency was associated with a 16% decrease in plasma HDL-cholesterol concentration and a 25% increase in aortic atherosclerosis. Thus, leukocyte-derived HL in CETPtgLDLr(-/-) mice has an atheroprotective role that may involve increased HDL levels.

Elevated expression of phospholipid transfer protein in bone marrow derived cells causes atherosclerosis

BACKGROUND

Phospholipid transfer protein (PLTP) is expressed by various cell types. In plasma, it is associated with high density lipoproteins (HDL). Elevated levels of PLTP in transgenic mice result in decreased HDL and increased atherosclerosis. PLTP is present in human atherosclerotic lesions, where it seems to be macrophage derived. The aim of the present study is to evaluate the atherogenic potential of macrophage derived PLTP.

METHODS AND FINDINGS

Here we show that macrophages from human PLTP transgenic mice secrete active PLTP. Subsequently, we performed bone marrow transplantations using either wild type mice (PLTPwt/wt), hemizygous PLTP transgenic mice (huPLTPtg/wt) or homozygous PLTP transgenic mice (huPLTPtg/tg) as donors and low density lipoprotein receptor deficient mice (LDLR-/-) as acceptors, in order to establish the role of PLTP expressed by bone marrow derived cells in diet-induced atherogenesis. Atherosclerosis was increased in the huPLTPtg/wt-->LDLR-/- mice (2.3-fold) and even further in the huPLTPtg/tg-->LDLR-/- mice (4.5-fold) compared with the control PLTPwt/wt-->LDLR-/- mice (both P<0.001). Plasma PLTP activity levels and non-HDL cholesterol were increased and HDL cholesterol decreased compared with controls (all P<0.01). PLTP was present in atherosclerotic plaques in the mice as demonstrated by immunohistochemistry and appears to co-localize with macrophages. Isolated macrophages from PLTP transgenic mice do not show differences in cholesterol efflux or in cytokine production. Lipopolysaccharide activation of macrophages results in increased production of PLTP. This effect was strongly amplified in PLTP transgenic macrophages.

CONCLUSIONS

We conclude that PLTP expression by bone marrow derived cells results in atherogenic effects on plasma lipids, increased PLTP activity, high local PLTP protein levels in the atherosclerotic lesions and increased atherosclerotic lesion size.

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.

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

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

Cholesterol accumulation is increased in macrophages of phospholipid transfer protein-deficient mice: normalization by dietary alpha-tocopherol supplementation

OBJECTIVE

Phospholipid transfer protein (PLTP) is a multifunctional, extracellular lipid transport protein that plays a major role in lipoprotein metabolism and atherosclerosis. Recent in vivo studies suggested that unlike systemic PLTP, macrophage-derived PLTP would be antiatherogenic. The present study aimed at characterizing the atheroprotective properties of macrophage-derived PLTP.

METHODS AND RESULTS

Peritoneal macrophages were isolated from PLTP-deficient and wild-type mice and their biochemical characteristics were compared. It is shown that macrophages isolated from PLTP-deficient mice have increased basal cholesterol content and accumulate more cholesterol in the presence of LDL compared with wild-type cells. Cholesterol parameters in macrophages of PLTP-deficient mice were normalized by dietary alpha-tocopherol supplementation.

CONCLUSIONS

The antiatherogenic properties of macrophage-derived PLTP are related at least in part to its ability to reduce cholesterol accumulation in macrophages through changes in the alpha-tocopherol content and oxidative status of the cells.

Expression of Human OSBP-Related Protein 1L in Macrophages Enhances Atherosclerotic Lesion Development in LDL Receptor–Deficient Mice

Objective— The purpose of this study was to assess the role of macrophage OSBP-related protein 1L (ORP1L) in the development of atherosclerosis.

Methods and Results— C57BL/6 mice overexpressing human ORP1L in macrophages driven by scavenger receptor A promoter were generated. Bone marrow (BM) of the mice was transplanted into LDLr −/− animals, and aortic root lesion area in the recipients was determined after Western-type diet feeding. The recipients of ORP1L BM displayed 2.1-fold increase ( P <0.001) in lesion size as compared with recipients of wild-type littermate BM. Macrophages of the ORP1L BM recipients showed a decrease in ABCG1 and APOE mRNAs and proteins, and an increase in PLTP message; also the plasma PLTP activity was elevated. The effect of ORP1L on cholesterol efflux was assessed using macrophages loaded with [ 3 H]cholesterol oleate-acLDL or labeled with [ 3 H]cholesterol. The ORP1L transgenic macrophages displayed 30% reduction ( P <0.01) in cholesterol efflux to HDL 2 , but not to apoA-I. ORP1L was shown to bind 25- and 22(R)-hydroxycholesterol, identifying it as an oxysterol binding protein. Furthermore, ORP1L attenuated the response of ABCG1 mRNA to 22(R)-hydroxycholesterol, the effect on ABCA1 being less pronounced.

Conclusions— The results demonstrate that macrophage ORP1L can act as a modulator of atherosclerotic lesion development and provide clues to the underlying mechanism.

Low phospholipid transfer protein (PLTP) is a risk factor for peripheral atherosclerosis

OBJECTIVE

Phospholipid transfer protein (PLTP) facilitates cholesterol efflux from cells, intravascular HDL remodelling and transfer of vitamin E and endotoxin. In humans, the relationship of PLTP to atherosclerosis is unknown. However, strong coronary risk factors like obesity, diabetes, cigarette smoking and inflammation increase circulating levels of active PLTP. The aim of the present, cross-sectional study was to analyze the relationship of PLTP to peripheral arterial disease, a marker of generalized atherosclerosis, independently of potentially confounding factors like obesity, diabetes and smoking.

METHODS

We performed a case control study in 153 patients with symptomatic peripheral arterial disease (PAD) and 208 controls free of vascular disease. Smokers and patients with diabetes mellitus were excluded. A lipoprotein-independent assay was used for measurement of circulating bioactive PLTP and an ELISA utilizing a monoclonal antibody was used to analyze PLTP mass.

RESULTS

PLTP activity was significantly decreased in patients with PAD 5.5 (4.6-6.4)(median (25th-75th percentile)) versus 5.9 (5.1-6.9) micromol/mL/h in controls (p=0.001). In contrast, PLTP mass was similar in patients with PAD 8.5 microg/mL (7.3-9.5) and in controls 8.3 microg/mL (6.9-9.7) (p=0.665). Multivariate logistic regression analysis revealed that PLTP activity is independently associated with the presence of PAD. PLTP activity was similar in patients with and without lipid-lowering drugs (p=0.396).

CONCLUSION

Our results show that in non-diabetic, non-smoking subjects low rather than high PLTP activity is a marker for the presence of peripheral arterial disease and that distribution of PLTP between high-activity and low-activity forms may be compromised in atherosclerosis.

Concerted actions of cholesteryl ester transfer protein and phospholipid transfer protein in type 2 diabetes: effects of apolipoproteins

PURPOSE OF REVIEW

Type 2 diabetes frequently coincides with dyslipidemia, characterized by elevated plasma triglycerides, low high-density lipoprotein cholesterol levels and the presence of small dense low-density lipoprotein particles. Plasma lipid transfer proteins play an essential role in lipoprotein metabolism. It is thus vital to understand their pathophysiology and determine which factors influence their functioning in type 2 diabetes.

RECENT FINDINGS

Cholesteryl ester transfer protein-mediated transfer is increased in diabetic patients and contributes to low plasma high-density lipoprotein cholesterol levels. Apolipoproteins A-I, A-II and E are components of the donor lipoprotein particles that participate in the transfer of cholesteryl esters from high-density lipoprotein to apolipoprotein B-containing lipoproteins. Current evidence for functional roles of apolipoproteins C-I, F and A-IV as modulators of cholesteryl ester transfer is discussed. Phospholipid transfer protein activity is increased in diabetic patients and may contribute to hepatic very low-density lipoprotein synthesis and secretion and vitamin E transfer. Apolipoprotein E could stimulate the phospholipid transfer protein-mediated transfer of surface fragments of triglyceride-rich lipoproteins to high-density lipoprotein, and promote high-density lipoprotein remodelling.

SUMMARY

Both phospholipid and cholesteryl ester transfer proteins are important in very low and high-density lipoprotein metabolism and display concerted actions in patients with type 2 diabetes.