An amphipathic helical region of the N-terminal barrel of phospholipid transfer protein is critical for ABCA1-dependent cholesterol efflux

Structure-based mechanism and inhibition of cholesteryl ester transfer protein

PURPOSE OF REVIEW

Cholesteryl ester transfer proteins (CETP) regulate plasma cholesterol levels by transferring cholesteryl esters (CEs) among lipoproteins. Lipoprotein cholesterol levels correlate with the risk factors for atherosclerotic cardiovascular disease (ASCVD). This article reviews recent research on CETP structure, lipid transfer mechanism, and its inhibition.

RECENT FINDINGS

Genetic deficiency in CETP is associated with a low plasma level of low-density lipoprotein cholesterol (LDL-C) and a profoundly elevated plasma level of high-density lipoprotein cholesterol (HDL-C), which correlates with a lower risk of atherosclerotic cardiovascular disease (ASCVD). However, a very high concentration of HDL-C also correlates with increased ASCVD mortality. Considering that the elevated CETP activity is a major determinant of the atherogenic dyslipidemia, i.e., pro-atherogenic reductions in HDL and LDL particle size, inhibition of CETP emerged as a promising pharmacological target during the past two decades. CETP inhibitors, including torcetrapib, dalcetrapib, evacetrapib, anacetrapib and obicetrapib, were designed and evaluated in phase III clinical trials for the treatment of ASCVD or dyslipidemia. Although these inhibitors increase in plasma HDL-C levels and/or reduce LDL-C levels, the poor efficacy against ASCVD ended interest in CETP as an anti-ASCVD target. Nevertheless, interest in CETP and the molecular mechanism by which it inhibits CE transfer among lipoproteins persisted. Insights into the structural-based CETP-lipoprotein interactions can unravel CETP inhibition machinery, which can hopefully guide the design of more effective CETP inhibitors that combat ASCVD. Individual-molecule 3D structures of CETP bound to lipoproteins provide a model for understanding the mechanism by which CETP mediates lipid transfer and which in turn, guide the rational design of new anti-ASCVD therapeutics.

Alterations of HDL's to piHDL's Proteome in Patients with Chronic Inflammatory Diseases, and HDL-Targeted Therapies

Chronic inflammatory diseases, such as rheumatoid arthritis, steatohepatitis, periodontitis, chronic kidney disease, and others are associated with an increased risk of atherosclerotic cardiovascular disease, which persists even after accounting for traditional cardiac risk factors. The common factor linking these diseases to accelerated atherosclerosis is chronic systemic low-grade inflammation triggering changes in lipoprotein structure and metabolism. HDL, an independent marker of cardiovascular risk, is a lipoprotein particle with numerous important anti-atherogenic properties. Besides the essential role in reverse cholesterol transport, HDL possesses antioxidative, anti-inflammatory, antiapoptotic, and antithrombotic properties. Inflammation and inflammation-associated pathologies can cause modifications in HDL's proteome and lipidome, transforming HDL from atheroprotective into a pro-atherosclerotic lipoprotein. Therefore, a simple increase in HDL concentration in patients with inflammatory diseases has not led to the desired anti-atherogenic outcome. In this review, the functions of individual protein components of HDL, rendering them either anti-inflammatory or pro-inflammatory are described in detail. Alterations of HDL proteome (such as replacing atheroprotective proteins by pro-inflammatory proteins, or posttranslational modifications) in patients with chronic inflammatory diseases and their impact on cardiovascular health are discussed. Finally, molecular, and clinical aspects of HDL-targeted therapies, including those used in therapeutical practice, drugs in clinical trials, and experimental drugs are comprehensively summarised.

Genomic Data Mining Reveals Abundant Uncharacterized Transporters in Coccidioides immitis and Coccidioides posadasii

Coccidioides immitis and Coccidioides posadasii are causative agents of coccidioidomycosis, commonly known as Valley Fever. The increasing Valley Fever cases in the past decades, the expansion of endemic regions, and the rising azole drug-resistant strains have underscored an urgent need for a better understanding of Coccidioides biology and new antifungal strategies. Transporters play essential roles in pathogen survival, growth, infection, and adaptation, and are considered as potential drug targets. However, the composition and roles of transport machinery in Coccidioides remain largely unknown. In this study, genomic data mining revealed an abundant, uncharacterized repertoire of transporters in Coccidioides genomes. The catalog included 1288 and 1235 transporter homologs in C. immitis and C. posadasii, respectively. They were further annotated to class, subclass, family, subfamily and range of substrates based on the Transport Classification (TC) system. They may play diverse roles in nutrient uptake, metabolite secretion, ion homeostasis, drug efflux, or signaling. This study represents an initial effort for a systems-level characterization of the transport machinery in these understudied fungal pathogens.

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.

The regulation of glucose and lipid homeostasis via PLTP as a mediator of BAT-liver communication

While brown adipose tissue (BAT) is well-recognized for its ability to dissipate energy in the form of heat, recent studies suggest multifaced roles of BAT in the regulation of glucose and lipid homeostasis beyond stimulating thermogenesis. One of the functions involves interorgan communication with metabolic organs, such as the liver, through BAT-derived secretory factors, a.k.a., batokine. However, the identity and the roles of such mediators remain insufficiently understood. Here, we employed proteomics and transcriptomics in human thermogenic adipocytes and identified previously unappreciated batokines, including phospholipid transfer protein (PLTP). We found that increased circulating levels of PLTP, via systemic or BAT-specific overexpression, significantly improve glucose tolerance and insulin sensitivity, increased energy expenditure, and decrease the circulating levels of cholesterol, phospholipids, and sphingolipids. Such changes were accompanied by increased bile acids in the circulation, which in turn enhances glucose uptake and thermogenesis in BAT. Our data suggest that PLTP is a batokine that contributes to the regulation of systemic glucose and lipid homeostasis as a mediator of BAT-liver interorgan communication.

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.

Lipid trafficking in cardiovascular disease

The reduction of plasma apolipoprotein B (apoB) containing lipoproteins has long been pursued as the main modifiable risk factor for the development of cardiovascular disease (CVD). This has led to an intense search for strategies aiming at reducing plasma apoB-lipoproteins, culminating in reduction of overall CV risk. Despite 3 decades of progress, CVD remains the leading cause of morbidity and mortality worldwide and, as such, new therapeutic targets are still warranted. Clinical and preclinical research has moved forward from the original concept, under which some lipids must be accumulated and other removed to achieve the ideal condition in disease prevention, into the concept that mechanisms that orchestrate lipid movement between lipoproteins, cells and organelles is equally involved in CVD. As such, this review scrutinizes potentially atherogenic changes in lipid trafficking and assesses the molecular mechanisms behind it. New developments in risk assessment and new targets for the mitigation of residual CVD risk are also addressed.

Rapamycin regulates cholesterol biosynthesis and cytoplasmic ribosomal proteins in hippocampus and temporal lobe of APP/PS1 mouse

As an inhibitor of the immune system and a longevity drug, rapamycin has been suggested as a treatment for Alzheimer's disease, although the underlying mechanisms remain to be clarified. To elucidate the mechanisms, we performed a high-throughput quantitative proteomics analysis and bioinformatics analysis of the changes in the proteome profiles of hippocampus and temporal lobe of wild-type mice, APP/PS1 mice and rapamycin-treated APP/PS1 mice (ProteomeXchange: PXD009540). Morris Water Maze tests were used to evaluate the effectiveness of rapamycin in APP/PS1 treatment and Western blot analysis was used to verify the proteomics data. The results of Morris Water Maze tests indicated that rapamycin improved the spatial learning and memory abilities of APP/PS1 mice. Proteome analysis identified 100 significantly changed (SC) proteins in hippocampus and 260 in temporal lobe in APP/PS1 mice. Among these, 57 proteins in hippocampus and 167 proteins in temporal lobe were rescued by rapamycin. STRING analysis indicated relatively more complicated protein interactions of AD-related rapamycin rescued proteins in temporal lobe. Pathway analysis showed that SC proteins in APP/PS1 mice were mainly enriched in cholesterol biosynthesis pathway and cytoplasmic ribosomal proteins. After rapamycin treatment, the expression of most proteins in these signaling pathways were reversed. Overall, our findings demonstrate that rapamycin may be an potential strategy which can effectively delays the progression of AD.

Plasma Phospholipid Transfer Protein Promotes Platelet Aggregation

It remains unclear whether plasma phospholipid transfer protein (PLTP) is involved in hyper-coagulation or hypo-coagulation. This study investigated the direct effect of PLTP on platelet aggregation and the underlying mechanism. Washed platelets from humans or mice and mouse platelet-rich plasma and human recombinant PLTP were isolated. PLTP is present in human platelets. We assessed adenosine diphosphate (ADP)-, collagen- and thrombin-induced platelet aggregation, phosphatidylserine externalization and photothrombosis-induced cerebral infarction in mice. PLTP over-expression increased platelet aggregation, while PLTP deficiency had the opposing reaction. Human recombinant PLTP increased both mouse and human platelet aggregation in a dose-dependent manner. Phosphatidylserine externalization provides a water/lipid surface for the interaction of coagulation factors, which accelerates thrombosis. Compared with wild-type controls, platelets from PLTP transgenic mice had significantly more phosphatidylserine on the exterior surface of the plasma membrane, whereas platelets from PLTP-deficient mice had significantly less phosphatidylserine on the surface, thus PLTP influences fibrinogen binding on the plasma membrane. Moreover, recombinant PLTP together with ADP significantly increased phosphatidylserine exposure on the plasma membrane of PLTP-deficient platelets, thereby increasing fibrinogen binding. PLTP over-expression significantly accelerated the incidence of photothrombosis-induced infarction in mice, whereas PLTP deficiency significantly reduced the frequency of infarction. We concluded that PLTP promotes phosphatidylserine externalization at the plasma membrane of platelets and accelerates ADP- or collagen-induced platelet aggregation. This effect plays an important role in the initiation of thrombin generation and platelet aggregation under sheer stress conditions. Thus, PLTP is involved in hyper-coagulation. Therefore, PLTP inhibition could be a novel approach for countering thrombosis.

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.

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

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

PhosphoLipid transfer protein (PLTP) exerts a direct pro-inflammatory effect on rheumatoid arthritis (RA) fibroblasts-like-synoviocytes (FLS) independently of its lipid transfer activity

Rheumatoid arthritis (RA) is a chronic inflammatory rheumatic disease with modification of lipids profile and an increased risk of cardiovascular events related to inflammation. Plasma phospholipid transfer protein (PLTP) exerts a lipid transfer activity through its active form. PLTP can also bind to receptors such as ATP-binding cassette transporter A1 (ABCA1). In addition to its role in lipoprotein metabolism and atherosclerosis, the latest advances came in support of a complex role of PLTP in the regulation of the inflammatory response, both with pro-inflammatory or anti-inflammatory properties. The aim of the present study was to decipher the role of PLTP in joint inflammation and to assess its relevance in the context of RA. PLTP expression was examined by western-blot and by immunochemistry. ABCA1 expression was analyzed by flow cytometry. Lipid transfer activity of PLTP and pro-inflammatory cytokines were measured in sera and synovial fluid (SF) from RA patients and controls (healthy subjects or osteoarthritis patients [OA]). FLS were treated with both lipid-transfer active form and inactive form of recombinant human PLTP. IL-8, IL-6, VEGF and MMP3 produced by FLS were assessed by ELISA, and proliferation by measuring ³H-Thymidine incorporation. RA synovial tissues showed higher PLTP staining than OA and PLTP protein levels were also significantly higher in RA-FLS. In addition, RA, unlike OA patients, displayed elevated levels of PLTP activity in SF, which correlated with pro-inflammatory cytokines. Both lipid-transfer active and inactive forms of PLTP significantly increased the production of cytokines and proliferation of FLS. ABCA1 was expressed on RAFLS and PLTP activated STAT3 pathway. To conclude, PLTP is highly expressed in the joints of RA patients and may directly trigger inflammation and FLS proliferation, independently of its lipid transfer activity. These results suggest a pro-inflammatory role for PLTP in RA.

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.

Overnutrition Determines LPS Regulation of Mycotoxin Induced Neurotoxicity in Neurodegenerative Diseases

Chronic neurodegenerative diseases are now associated with obesity and diabetes and linked to the developing and developed world. Interests in healthy diets have escalated that may prevent neurodegenerative diseases such as Parkinson's and Alzheimer's disease. The global metabolic syndrome involves lipoprotein abnormalities and insulin resistance and is the major disorder for induction of neurological disease. The effects of bacterial lipopolysaccharides (LPS) on dyslipidemia and NAFLD indicate that the clearance and metabolism of fungal mycotoxins are linked to hypercholesterolemia and amyloid beta oligomers. LPS and mycotoxins are associated with membrane lipid disturbances with effects on cholesterol interacting proteins, lipoprotein metabolism, and membrane apo E/amyloid beta interactions relevant to hypercholesterolemia with close connections to neurological diseases. The influence of diet on mycotoxin metabolism has accelerated with the close association between mycotoxin contamination from agricultural products such as apple juice, grains, alcohol, and coffee. Cholesterol efflux in lipoproteins and membrane cholesterol are determined by LPS with involvement of mycotoxin on amyloid beta metabolism. Nutritional interventions such as diets low in fat/carbohydrate/cholesterol have become of interest with relevance to low absorption of lipophilic LPS and mycotoxin into lipoproteins with rapid metabolism of mycotoxin to the liver with the prevention of neurodegeneration.

Dysfunctional High-Density Lipoprotein: An Innovative Target for Proteomics and Lipidomics

High-Density Lipoprotein-Cholesterol (HDL-C) is regarded as an important protective factor against cardiovascular disease, with abundant evidence of an inverse relationship between its serum levels and risk of cardiovascular disease, as well as various antiatherogenic, antioxidant, and anti-inflammatory properties. Nevertheless, observations of hereditary syndromes featuring scant HDL-C concentration in absence of premature atherosclerotic disease suggest HDL-C levels may not be the best predictor of cardiovascular disease. Indeed, the beneficial effects of HDL may not depend solely on their concentration, but also on their quality. Distinct subfractions of this lipoprotein appear to be constituted by specific protein-lipid conglomerates necessary for different physiologic and pathophysiologic functions. However, in a chronic inflammatory microenvironment, diverse components of the HDL proteome and lipid core suffer alterations, which propel a shift towards a dysfunctional state, where HDL-C becomes proatherogenic, prooxidant, and proinflammatory. This heterogeneity highlights the need for further specialized molecular studies in this aspect, in order to achieve a better understanding of this dysfunctional state; with an emphasis on the potential role for proteomics and lipidomics as valuable methods in the search of novel therapeutic approaches for cardiovascular disease.

Influence of Isoforms and Carboxyl-Terminal Truncations on the Capacity of Apolipoprotein E To Associate with and Activate Phospholipid Transfer Protein

Phospholipid transfer protein (PLTP), a main protein in lipid and lipoprotein metabolism, exists in high-activity (HA-PLTP) and low-activity (LA-PLTP) forms in human plasma. Proper phospholipid transfer activity of PLTP is modulated by interactions with various apolipoproteins (apo) including apoE. The domains of apoE involved in interactions with PLTP are not known. Here we analyzed the capacity of recombinant apoE isoforms and apoE4 mutants with progressive carboxyl-terminal deletions to bind to and activate HA-PLTP and LA-PLTP. Our analyses demonstrated that lipid-free apoE isoforms bind to both HA-PLTP and LA-PLTP, resulting in phospholipid transfer activation, with apoE3 inducing the highest PLTP activation. The isoform-specific differences in apoE/PLTP binding and PLTP activation were abolished following apoE lipidation. Lipid-free apoE4[Δ(260-299)], apoE4[Δ(230-299)], apoE4[Δ(203-299)], and apoE4[Δ(186-299)] activated HA-PLTP by 120-160% compared to full-length apoE4. Lipid-free apoE4[Δ(186-299)] also activated LA-PLTP by 85% compared to full-length apoE4. All lipidated truncated apoE4 forms displayed a similar effect on HA-PLTP and LA-PLTP activity as full-length apoE4. Strikingly, lipid-free or lipidated full-length apoE4 and apoE4[Δ(186-299)] demonstrated similar binding capacity to LA-PLTP and HA-PLTP. Biophysical studies showed that the carboxyl-terminal truncations of apoE4 resulted in small changes of the structural or thermodynamic properties of lipidated apoE4, that were much less pronounced compared to changes observed previously for lipid-free apoE4. Overall, our findings show an isoform-dependent binding to and activation of PLTP by lipid-free apoE. Furthermore, the domain of apoE4 required for PLTP activation resides within its amino-terminal 1-185 region. The apoE/PLTP interactions can be modulated by the conformation and lipidation state of apoE.

Inhibition of thrombin generation in human plasma by phospholipid transfer protein

Background

Plasma phospholipid transfer protein (PLTP) transfers lipids between donors and acceptors (e.g., from HDL to VLDL) and modulates lipoprotein composition, size, and levels. No study has reported an assessment of the effects of PLTP on blood clotting reactions, such as reflected in thrombin generation assays, or on the association of venous thrombosis (VTE) risk with PLTP activity.

Methods

The in vitro effects of PLTP on blood coagulation reactions and the correlations between plasma PLTP activity levels and VTE were studied.

Results

Recombinant (r) PLTP concentration-dependently inhibited plasma thrombin generation and factor XII-dependent kallikrein generation when sulfatide was used to stimulate factor XII autoactivation in plasma. However, rPLTP did not inhibit thrombin generation in plasma induced by factor XIa or tissue factor, implicating an effect of PLTP on contact activation reactions. In purified systems, rPLTP inhibited factor XII autoactivation stimulated by sulfatide in the presence of VLDL. In surface plasmon resonance studies, purified factor XII bound to immobilized rPLTP, implying that rPLTP inhibits factor XII-dependent contact activation by binding factor XII in the presence of lipoproteins. Analysis of plasmas from 40 male patients with unprovoked VTE and 40 matched controls indicated that low PLTP lipid transfer activity (≤25th percentile) was associated with an increased risk of VTE after adjustment for body mass index, plasma lipids, and two known thrombophilic genetic risk factors.

Conclusion

These data imply that PLTP may be an antithrombotic plasma protein by inhibiting generation of prothrombotic factor XIIa in the presence of VLDL. This newly discovered anticoagulant activity of PLTP merits further clinical and biochemical studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12959-015-0054-0) contains supplementary material, which is available to authorized users.

Phospholipid transfer protein plays a major role in the initiation of apolipoprotein B-containing lipoprotein assembly in mouse primary hepatocytes

In this study, we tested the hypothesis that phospholipid transfer protein (PLTP) is a plausible mediator of phospholipid (PL) transfer to the N-terminal 1000 residues of apoB (apoB:1000) leading to the initiation of apoB-containing lipoprotein assembly. To this end, primary hepatocytes from wild type (WT) and PLTP knock-out (KO) mice were transduced with adenovirus-apoB:1000 with or without co-transduction with adenovirus-PLTP, and the assembly and secretion of apoB:1000-containing lipoproteins were assessed. PLTP deficiency resulted in a 65 and 72% reduction in the protein and lipid content, respectively, of secreted apoB:1000-containing lipoproteins. Particles secreted by WT hepatocytes contained 69% PL, 9% diacylglycerol (DAG), and 23% triacylglycerol (TAG) with a stoichiometry of 46 PL, 6 DAG, and 15 TAG molecules per apoB:1000. PLTP absence drastically altered the lipid composition of apoB:1000 lipoproteins; these particles contained 46% PL, 13% DAG, and 41% TAG with a stoichiometry of 27 PL, 10 DAG, and 23 TAG molecules per apoB:1000. Reintroduction of Pltp gene into PLTP-KO hepatocytes stimulated the lipidation and secretion of apoB:1000-containing lipoproteins by ∼3-fold; the lipid composition and stoichiometry of these particles were identical to those secreted by WT hepatocytes. In contrast to the WT, apoB:1000 in PLTP-KO hepatocytes was susceptible to intracellular degradation predominantly in the post-endoplasmic reticulum, presecretory compartment. Reintroduction of Pltp gene into PLTP-KO hepatocytes restored the stability of apoB:1000. These results provide compelling evidence that in hepatocytes initial recruitment of PL by apoB:1000 leading to the formation of the PL-rich apoB-containing initiation complex is mediated to a large extent by PLTP.

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.

Impact of phospholipid transfer protein on nascent high-density lipoprotein formation and remodeling

OBJECTIVE

Phospholipid transfer protein (PLTP), which binds phospholipids and facilitates their transfer between lipoproteins in plasma, plays a key role in lipoprotein remodeling, but its influence on nascent high-density lipoprotein (HDL) formation is not known. The effect of PLTP overexpression on apolipoprotein A-I (apoA-I) lipidation by primary mouse hepatocytes was investigated.

APPROACH AND RESULTS

Overexpression of PLTP through an adenoviral vector markedly affected the amount and size of lipidated apoA-I species that were produced in hepatocytes in a dose-dependent manner, ultimately generating particles that were <7.1 nm but larger than lipid-free apoA-I. These <7.1-nm small particles generated in the presence of overexpressed PLTP were incorporated into mature HDL particles more rapidly than apoA-I both in vivo and in vitro and were less rapidly cleared from mouse plasma than lipid-free apoA-I. The <7.1-nm particles promoted both cellular cholesterol and phospholipid efflux in an ATP-binding cassette transporter A1-dependent manner, similar to apoA-I in the presence of PLTP. Lipid-free apoA-I had a greater efflux capacity in the presence of PLTP than in the absence of PLTP, suggesting that PLTP may promote ATP-binding cassette transporter A1-mediated cholesterol and phospholipid efflux. These results indicate that PLTP alters nascent HDL formation by modulating the lipidated species and by promoting the initial process of apoA-I lipidation.

CONCLUSIONS

Our findings suggest that PLTP exerts significant effects on apoA-I lipidation and nascent HDL biogenesis in hepatocytes by promoting ATP-binding cassette transporter A1-mediated lipid efflux and the remodeling of nascent HDL particles.

Phospholipid transfer protein is expressed in cerebrovascular endothelial cells and involved in high density lipoprotein biogenesis and remodeling at the blood-brain barrier

Phospholipid transfer protein (PLTP) is a key protein involved in biogenesis and remodeling of plasma HDL. Several neuroprotective properties have been ascribed to HDL. We reported earlier that liver X receptor (LXR) activation promotes cellular cholesterol efflux and formation of HDL-like particles in an established in vitro model of the blood-brain barrier (BBB) consisting of primary porcine brain capillary endothelial cells (pBCEC). Here, we report PLTP synthesis, regulation, and its key role in HDL metabolism at the BBB. We demonstrate that PLTP is highly expressed and secreted by pBCEC. In a polarized in vitro model mimicking the BBB, pBCEC secreted phospholipid-transfer active PLTP preferentially to the basolateral ("brain parenchymal") compartment. PLTP expression levels and phospholipid transfer activity were enhanced (up to 2.5-fold) by LXR activation using 24(S)-hydroxycholesterol (a cerebral cholesterol metabolite) or TO901317 (a synthetic LXR agonist). TO901317 administration elevated PLTP activity in BCEC from C57/BL6 mice. Preincubation of HDL3 with human plasma-derived active PLTP resulted in the formation of smaller and larger HDL particles and enhanced the capacity of the generated HDL particles to remove cholesterol from pBCEC by up to 3-fold. Pre-β-HDL, detected by two-dimensional crossed immunoelectrophoresis, was generated from HDL3 in pBCEC-derived supernatants, and their generation was markedly enhanced (1.9-fold) upon LXR activation. Furthermore, RNA interference-mediated PLTP silencing (up to 75%) reduced both apoA-I-dependent (67%) and HDL3-dependent (30%) cholesterol efflux from pBCEC. Based on these findings, we propose that PLTP is actively involved in lipid transfer, cholesterol efflux, HDL genesis, and remodeling at the BBB.

Cathepsin G degradation of phospholipid transfer protein (PLTP) augments pulmonary inflammation

Phospholipid transfer protein (PLTP) regulates phospholipid transport in the circulation and is highly expressed within the lung epithelium, where it is secreted into the alveolar space. Since PLTP expression is increased in chronic obstructive pulmonary disease (COPD), this study aimed to determine how PLTP affects lung signaling and inflammation. Despite its increased expression, PLTP activity decreased by 80% in COPD bronchoalveolar lavage fluid (BALF) due to serine protease cleavage, primarily by cathepsin G. Likewise, PLTP BALF activity levels decreased by 20 and 40% in smoke-exposed mice and in the media of smoke-treated small airway epithelial (SAE) cells, respectively. To assess how PLTP affected inflammatory responses in a lung injury model, PLTP siRNA or recombinant protein was administered to the lungs of mice prior to LPS challenge. Silencing PLTP at baseline caused a 68% increase in inflammatory cell infiltration, a 120 and 340% increase in ERK and NF-κB activation, and increased MMP-9, IL1β, and IFN-γ levels after LPS treatment by 39, 140, and 190%, respectively. Conversely, PLTP protein administration countered these effects in this model. Thus, these findings establish a novel anti-inflammatory function of PLTP in the lung and suggest that proteolytic cleavage of PLTP by cathepsin G may enhance the injurious inflammatory responses that occur in COPD.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Liver-specific phospholipid transfer protein deficiency reduces high-density lipoprotein and non-high-density lipoprotein production in mice

OBJECTIVE

The liver is one of the critical organs for lipoprotein metabolism and a major source for phospholipid transfer protein (PLTP) expression. The effect of liver-specific PLTP deficiency on plasma lipoprotein production and metabolism in mice was investigated.

APPROACH AND RESULTS

We created a liver-specific PLTP-deficient mouse model. We measured plasma high-density lipoprotein (HDL) and apolipoprotein B (apoB)-containing lipoprotein (or non-HDL) levels and their production rates. We found that hepatic ablation of PLTP leads to a significant decrease in plasma PLTP activity, HDL lipids, non-HDL lipids, apoAI, and apoB levels. In addition, nuclear magnetic resonance examination of lipoproteins showed that the deficiency decreases HDL and apoB-containing lipoprotein particle numbers, as well as very low-density lipoprotein particle size, which was confirmed by electron microscopy. Moreover, HDL particles from the deficient mice are lipid-poor ones. To unravel the mechanism, we evaluated the apoB and triglyceride production rates. We found that hepatic PLTP deficiency significantly decreases apoB and triglyceride secretion rates. To investigate the role of liver PLTP on HDL production, we set up primary hepatocyte culture studies and found that the PLTP-deficient hepatocytes produce less nascent HDL. Furthermore, we found that exogenous PLTP promotes nascent HDL production through an ATP-binding cassette A 1-mediated pathway.

CONCLUSIONS

Liver-specific PLTP deficiency significantly reduces plasma HDL and apoB-containing lipoprotein levels. Reduction of production rates of both particles is one of the mechanisms.

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.

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.

Regulation of ABCA1 functions by signaling pathways

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that protects cardiovascular disease by at least two mechanisms: by export of excess cholesterol from cells and by suppression of inflammation. ABCA1 exports cholesterol and phospholipids from cells by multiple steps that involve forming cell surface lipid domains, binding of apolipoproteins to ABCA1, activating signaling pathways, and solubilizing these lipids by apolipoproteins. ABCA1 executes its anti-inflammatory effect by modifying cell membrane lipid rafts and directly activating signaling pathways. The interaction of apolipoproteins with ABCA1 activates multiple signaling pathways, including Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3), protein kinase A, Rho family G protein CDC42 and protein kinase C. Activating protein kinase A and Rho family G protein CDC42 regulates ABCA1-mediated lipid efflux, activating PKC stabilizes ABCA1 protein, and activating JAK2/STAT3 regulates both ABCA1-mediated lipid efflux and anti-inflammation. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. Targeting ABCA1 receptor-like property using agonists for ABCA1 protein could become a promising new therapeutic target for increasing ABCA1 function and treating cardiovascular 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).

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).

Different phospholipid transfer protein complexes contribute to the variation in plasma PLTP specific activity

Phospholipid transfer protein (PLTP) facilitates the transfer of phospholipids among lipoproteins. Over half of the PLTP in human plasma has been found to have little phospholipid transfer activity (inactive PLTP). We recently observed that plasma PLTP specific activity is inversely correlated with high-density lipoprotein (HDL) level and particle size in healthy adults. The purpose of this study was to evaluate the factors that contribute to the variation in plasma PLTP specific activity. Analysis of the specific activity of PLTP complexes in nine plasma samples from healthy adults revealed two clusters of inactive PLTP complexes with mean molecular weights (MW) of 342kDa and 146kDa. The large and small inactive PLTP complexes represented 52±8% (range 39-63%) and 8±8% (range 1-28%) of the plasma PLTP, respectively. Active PLTP complexes had a mean MW of 207kDa and constituted 40±6% (range 33-50%) of the plasma PLTP. The specific activity of active PLTP varied from 16 to 32μmol/μg/h. These data demonstrate for the first time the existence of small inactive plasma PLTP complexes. Variation in the amount of the two clusters of inactive PLTP complexes and the specific activity of the active PLTP contribute to the variation in plasma PLTP specific activity.

Proposed mechanism for lipoprotein remodelling in the brain

Lipoprotein remodelling in the periphery has been extensively studied. For example, the processing of nascent apoAI particles to cholesterol-loaded HDL lipoproteins during reverse cholesterol transport involves a series of enzymes, transporters in peripheral tissue, as well as other apolipoproteins and lipoproteins. These extensive modifications and interconversions are well defined. Here, we present the hypothesis that a similar process occurs within the blood brain barrier (BBB) via glia-secreted lipid-poor apoE particles undergoing remodelling to become mature central nervous system (CNS) lipoproteins. We further pose several pressing issues and future directions for the study of lipoproteins in the brain.

Reduction of HDL levels lowers plasma PLTP and affects its distribution among lipoproteins in mice

Phospholipid transfer protein (PLTP) is associated with HDL particles in plasma, where it transfers phospholipids between lipoproteins and remodels HDL particles. Tangier disease patients, with a mutated ABCA1 transporter, have extremely low plasma HDL concentration and reduced PLTP activity levels, a phenotype that is also observed in mice lacking ABCA1. We investigated whether low HDL levels and low PLTP activity are mechanistically related. Firstly, we studied PLTP expression and distribution among lipoproteins in mice lacking ABCA1 (ABCA1(-/-)). Parallel to the strong reduction in PLTP activity in plasma of ABCA1(-/-) mice, decreased PLTP protein levels were observed. Neither PLTP synthesis in liver or macrophages nor the ability of the macrophages to secrete PLTP were impaired in ABCA1(-/-) mice. However, the PLTP activity level in the medium of cultured macrophages was determined by HDL levels in the medium. PLTP was associated with HDL particles in wild type mice, whereas in ABCA1(-/-) mice, PLTP was associated with VLDL and LDL particles. Secondly, we treated different mouse models with varying plasma HDL and PLTP levels (wild type, ABCA1(-/-), apoE(-/-) and PLTPtg mice, overexpressing human PLTP) with a synthetic LXR ligand, and investigated the relationship between LXR-mediated PLTP induction and HDL levels in plasma. Plasma PLTP activity in wild type mice was induced 5.6-fold after LXR activation, whereas in ABCA1(-/-), apoE(-/-) and PLTPtg mice, all having reduced HDL levels, induction of PLTP activity was 2.4- , 3.2- and 2.0-fold, respectively. The less pronounced PLTP induction in these mice compared to wild type mice was not caused by a decreased PLTP gene expression in the liver or macrophages. Our findings indicate that the extent of LXR-mediated PLTP induction depends on plasma HDL levels. In conclusion, we demonstrate that ABCA1 deficiency in mice affects plasma PLTP level and distribution through an indirect effect on HDL metabolism. In addition, we show that the extent of LXR-mediated PLTP induction is HDL-dependent. These findings indicate that plasma HDL level is an important regulator of plasma PLTP and might play a role in the stabilization of PLTP in plasma.

The cell cholesterol exporter ABCA1 as a protector from cardiovascular disease and diabetes

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that exports cholesterol from cells and suppresses macrophage inflammation. ABCA1 exports cholesterol by a multistep pathway that involves forming cell-surface lipid domains, solubilizing these lipids by apolipoproteins, binding of apolipoproteins to ABCA1, and activating signaling processes. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. ABCA1 transcription is highly induced by sterols, and its expression and activity are regulated post-transcriptionally by diverse processes. ABCA1 mutations can reduce plasma HDL levels, accelerate cardiovascular disease, and increase the risk for type 2 diabetes. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels, inflammation, atherogenesis, and pancreatic beta cell function. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages, raising the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become a promising new therapeutic target for treating cardiovascular disease and diabetes.