Lipolysis Of Triglyceride-rich Lipoproteins Research Articles

Induction of ATF3 Gene Network by Triglyceride-Rich Lipoprotein Lipolysis Products Increases Vascular Apoptosis and Inflammation

Elevation of triglyceride-rich lipoproteins (TGRLs) contributes to the risk of atherosclerotic cardiovascular disease. Our work has shown that TGRL lipolysis products in high physiological to pathophysiological concentrations cause endothelial cell injury; however, the mechanisms remain to be delineated. We analyzed the transcriptional signaling networks in arterial endothelial cells exposed to TGRL lipolysis products. When human aortic endothelial cells in culture were exposed to TGRL lipolysis products, activating transcription factor 3 (ATF3) was identified as a principal response gene. Induction of ATF3 mRNA and protein was confirmed by quantitative reverse-transcription polymerase chain reaction and Western blot respectively. Immunofluorescence analysis showed that ATF3 accumulated in the nuclei of cells treated with lipolysis products. Nuclear expression of phosphorylated c-Jun N-terminal kinase (JNK), previously shown to be an initiator of the ATF3 signaling cascade, also was demonstrated. Small interfering RNA (siRNA)-mediated inhibition of ATF3 blocked lipolysis products-induced transcription of E-selectin and interleukin-8, but not interleukin-6 or nuclear factor-κB. c-Jun, a downstream protein in the JNK pathway, was phosphorylated, whereas expression of nuclear factor-κB-dependent JunB was downregulated. Additionally, JNK siRNA suppressed ATF3 and p-c-Jun protein expression, suggesting that JNK is upstream of the ATF3 signaling pathway. In vivo studies demonstrated that infusion of TGRL lipolysis products into wild-type mice induced nuclear ATF3 accumulation in carotid artery endothelium. ATF3(-/-) mice were resistant to vascular apoptosis precipitated by treatment with TGRL lipolysis products. Also peripheral blood monocytes isolated from postprandial humans had increased ATF3 expression as compared with fasting monocytes. This study demonstrates that TGRL lipolysis products activate ATF3-JNK transcription factor networks and induce endothelial cells inflammatory response.

Abstract 547: TGFß Response Coupled to Stress Signaling by TGRL Lipolysis in Vascular Endothelial Cells

Elevation of circulating triglyceride-rich lipoproteins (TGRL) in the postprandial state is associated with increased endothelial cell inflammation and dysfunction potentially contributing to atherosclerosis. Previous studies from our laboratory showed that lipolysis products of TGRL induced endothelial cell barrier permeability, up-regulation of cytokines and adhesion molecules, and apoptosis. Gene array studies implicated the Stress Associated Protein Kinase and c-Jun N-terminal kinase (SAPK/JNK) pathway and identified the transcription factor activating transcription factor 3 (ATF3) as highly induced in this process. Additional studies demonstrated activation of second messengers for transforming growth factor beta (TGFβ) signaling (SMAD2/SMAD4) in response to lipolysis product treatment. In the present study, we asked whether TGFβ signaling was implicated in the induction of apoptosis by lipolysis products and whether there was a linkage between TGFβ receptor activation and pro-inflammatory responses. siRNA knockdown of ATF3 transcription abrogated both IL-8 and E-selectin up-regulation in lipolysis product-treated human aortic endothelial cells (HAEC). In addition, ATF3 knockdown prevented activation of caspase 3/7 and induction of p53, markers of apoptosis. Inhibition of the TGFβ Type I activin receptor-like kinase (ALK) receptors (ALKs 4, 5 and 7) by SB 431542 (10 μM) also inhibited inflammatory up-regulation, ATF3 induction and apoptosis. Both ATF3 knockdown and TGFβ receptor blockade prevented c-Jun phosphorylation, a co-activating transcription factor with ATF3 of AP-1 transcription sites. Our findings implicate TGFβ signaling in the inflammatory and apoptotic responses to TGRL lipolysis and suggest a complex interaction between TGFβ receptors and the SAPK subset of MAP Kinases.

Abstract 471: Triglyceride-rich Lipoprotein Lipolysis Products Induce Adipose Differentiation Related Protein Expression and Proatherogenic Cytokine Secretion in Monocytes

Elevated blood triglyceride levels are associated with an increased risk of atherosclerotic cardiovascular disease. Blood triglycerides are contained within very low density lipoproteins (VLDL) and chylomicrons, which are referred to as triglyceride-rich lipoproteins (TGRL). The enzyme lipoprotein lipase is responsible for the hydrolysis of these triglycerides, primarily into free fatty acids and glycerol. Our lab has shown that these lipolysis products induce lipid droplet formation in monocytes and can induce a proinflammatory state in multiple cell types. Other investigators have linked adipose differentiation related protein (ADRP, also known as perilipin 2 or adipophilin) to lipid droplet formation in monocyte-derived macrophage foam cells. The objectives of this study are to further understand the effects of TGRL lipolysis products on the formation of lipid droplets in monocytes and monocyte activation. We hypothesize that TGRL lipolysis products increase the expression of ADRP and the secretion of proatherogenic cytokines in THP-1 monocytes. In this study, we analyzed the global gene expression in THP-1 monocytes exposed to TGRL lipolysis products. ADRP was identified as a principal response lipid droplet associated gene induced by exposure to TGRL lipolysis products when compared to the control treatment. Induction of protein expression was analyzed by western blot. The TGRL lipolysis product treatment was also found to induce genes associated with atherosclerosis and monocyte adherence to endothelium including growth differentiation factor 15 (GDF15) and interleukin 8 (IL-8) when compared to the control treatment. We measured the secretion of GDF15 and IL-8 by ELISA. Our results indicate that mRNA and protein expression of ADRP and secretion of proatherogenic cytokines (GDF15 and IL-8) were increased in monocytes after exposure to TGRL lipolysis products. In conclusion, our study demonstrates that TGRL lipolysis products induce the expression of ADRP and the secretion of proatherogenic cytokines in monocytes. These results may indicate a role for TGRL lipolysis products in the increased risk of atherosclerotic cardiovascular disease for individuals with elevated blood triglyceride levels.

Palmitic acid induces but docosahexaenoic acid (DHA) inhibits inflammasome‐mediated secretion of IL‐1β in monocytes by modulating the activation of Toll‐like receptor 2 (TLR2)

Activation of proinflammatory pathways in monocytes leads to enhanced inflammation in various tissues which in turn can promote the development of many chronic diseases. Unlike other cytokines, IL‐1β production is regulated at both transcriptional and posttranslational levels. TLR activation induces expression of pro‐ IL‐1β that needs to be processed by activated inflammasome to release bioactive IL‐1β. High saturated fat consumption is associated with increased postprandial inflammation. However the mechanism remains unclear. Thus, we determined whether 1) a major dietary saturated fatty acid palmitic acid (C16), as well as lipolysis products of postprandial triglyceride‐rich lipoproteins (TGRL) containing palmitic acid as the major fatty acid, induce IL‐1β expression and stimulate inflammasome‐mediated IL‐1β secretion in monocytes, 2) whether this process is TLR‐mediated and, 3) whether DHA inhibits this process. Results show that C16 and TGRL lipolysis products induce but DHA inhibits the expression and secretion of IL‐1β by reciprocally modulating TLR2 activation as assessed by recruitment of TLR2 and adaptor molecules into lipid rafts (faithful readout for direct activation of TLR2). Together, these results suggest that inflammasome‐mediated inflammatory responses in blood monocytes can be dynamically modulated by types of dietary fat we consume.Grant Funding Source : USDA‐ARS WHNRC Program Funds and NIHDK 064007

Reciprocal Metabolic Perturbations in the Adipose Tissue and Liver of GPIHBP1-Deficient Mice

Gpihbp1-deficient (Gpihbp1-/-) mice lack the ability to transport lipoprotein lipase to the capillary lumen, resulting in mislocalization of lipoprotein lipase within tissues, defective lipolysis of triglyceride-rich lipoproteins, and chylomicronemia. We asked whether GPIHBP1 deficiency and mislocalization of catalytically active lipoprotein lipase would alter the composition of triglycerides in adipose tissue or perturb the expression of lipid biosynthetic genes. We also asked whether perturbations in adipose tissue composition and gene expression, if they occur, would be accompanied by reciprocal metabolic changes in the liver. The chylomicronemia in Gpihbp1-/- mice was associated with reduced levels of essential fatty acids in adipose tissue triglycerides and increased expression of lipid biosynthetic genes. The liver exhibited the opposite changes: increased levels of essential fatty acids in triglycerides and reduced expression of lipid biosynthetic genes. Defective lipolysis in Gpihbp1-/- mice causes reciprocal metabolic perturbations in adipose tissue and liver. In adipose tissue, the essential fatty acid content of triglycerides is reduced and lipid biosynthetic gene expression is increased, whereas the opposite changes occur in the liver.

GPIHBP1 C89F Neomutation and Hydrophobic C-Terminal Domain G175R Mutation in Two Pedigrees with Severe Hyperchylomicronemia

GPIHBP1 is a new endothelial binding site for lipoprotein lipase (LPL), the key enzyme for intravascular lipolysis of triglyceride-rich lipoproteins (TGRL). We have identified two new missense mutations of the GPIHBP1 gene, C89F and G175R, by systematic sequencing in a cohort of 376 hyperchylomicronemic patients without mutations on the LPL, APOC2, or APOA5 gene. Phenotypic expression and functional consequences of these two mutations were studied. We performed clinical and genotypic studies of probands and their families. GPIHBP1 functional alterations were studied in CHO pgsA-745 transfected cells. Probands are an adult with a homozygous G175R mutation and a child with a hemizygous C89F neomutation and a deletion of the second allele. C89F mutation was associated with a C14F signal peptide polymorphism on the same haplotype. Both patients had resistant hyperchylomicronemia, low LPL activity, and history of acute pancreatitis. In CHO pgsA-745 cells, both G175R and C14F variants reduce the expression of GPIHBP1 at the cell surface. C89F mutation is responsible for a drastic LPL-binding defect to GPIHBP1. C14F may further potentiate C89F effect. The emergence of hyperchylomicronemia in the generation after a neomutation further establishes a critical role for GPIHBP1 in TGRL physiopathology in humans. Our results highlight the crucial role of C65-C89 disulfide bond in LPL binding by GPIHBP1 Ly6 domain. Furthermore, we first report a mutation of the hydrophobic C-terminal domain that impairs GPIHBP1 membrane targeting.

Novel aspects of HDL level and function in a clinical setting

HDL is composed of heterogeneous particles ranging between 7 and 14 nm. ApoA1, the main HDL apolipoprotein, is synthesized in hepatic and intestinal cells and is a determinant of the formation and stability of HDL and the two main HDL functions: cholesterol esterification and reverse cholesterol transportation [1]. HDL is secreted by the liver as disk-shaped particles constituted by ApoA1, phospholipids and unesterified cholesterol. Nascent HDL becomes progressively spherical by cholesterol esterification. HDL also originates in the plasma compartment, by accretion of ApoA1 to phospholipids and unesterified cholesterol derived from lipolysis of triglyceride-rich lipoproteins [1]. Continuous exchange of lipids and proteins between HDL and the other lipoproteins and cholesterol esterification keeps HDL particles in constant remodeling. Lipid exchange is mediated by cholesteryl ester transfer protein (CETP), mainly for cholesteryl esters and triglycerides and phospholipid transfer protein mainly for phospholipids [2]. Reverse cholesterol transportation begins with the binding of HDL ApoA1 to the ABCA1 protein complex and the transfer of unesterified cholesterol from the cells to HDL. The transfer of cholesterol to nascent HDL is facilitated by ABCA1, whereas ABCAG1 shuttles cholesterol to mature HDL particles. Subsequently, reverse cholesterol transportation is driven by the cholesterol esterification process in HDL particles that are taken up by the liver by SR-BI receptors or transferred to other lipoproteins that are also taken up by the liver for excretion in bile [2].

Phospholipidation of HDL—How Much Is Too Much?

Phospholipid transfer protein (PLTP)2 is a secreted lipid transfer protein that mediates the exchange of phospholipids between lipoproteins in the circulation. By shuttling phospholipid surface remnants generated during triglyceride-rich lipoprotein lipolysis to HDL, PLTP plays an important role in HDL maturation. In addition to forming the phospholipid shell of the particle, PLTP-derived phospholipids serve as a substrate for lecithin-cholesteryl acyltransferase, which generates cholesteryl esters to form the neutral lipid core. PLTP also promotes remodeling and rearrangement of HDL in a process known as “HDL conversion.” In the presence of human apolipoprotein AI (apo AI), PLTP promotes the fusion of smaller HDL3 particles to generate larger HDL2 particles, along with the simultaneous shedding of lipid-poor apo AI. In doing so, PLTP-mediated HDL conversion promotes the regeneration of pre-β HDL. Our references are limited because of space limitations; for a more detailed review of the PLTP literature, please consult one of the excellent review articles on this topic (1, 2). PLTP-deficient mice have major reductions in HDL cholesterol (HDL-C) concentrations and a shift toward smaller HDL particle size that are reflective of the critical role for this protein in HDL metabolism. Paradoxically, substantial overproduction of PLTP in mice also reduces HDL-C concentrations by destabilizing the particles and promoting their catabolism. In the presence of human apo AI, however, modest overproduction of human PLTP has been shown to increase HDL-C and pre-β HDL values, possibly by providing phospholipid as a lecithin-cholesteryl acyltransferase substrate and promoting HDL maturation. In addition to its effects on HDL metabolism, PLTP production has profound effects on the concentrations of apo B lipoproteins. PLTP-deficient mice have lower concentrations of apo B lipoproteins, and small-molecule inhibition of PLTP decreases apo B secretion from hepatocytes. It appears that hepatic PLTP plays an important role in phospholipidation …

GPIHBP1 and the processing of triglyceride-rich lipoproteins

GPIHBP1 is a new addition to a group of proteins required for the lipolysis of triglyceride-rich lipoproteins. GPIHBP1 contains an acidic domain and an Ly6 domain with ten cysteines. GPIHBP1 binds lipoprotein lipase (LPL) avidly and likely tethers LPL to the luminal surface of capillaries.Inactivation of Gpihbp1 in mice is associated with milky plasma and severe chylomicronemia, even on a low-fat chow diet. Recently, four missense mutations in GPIHBP1 were identified in humans with severe chylomicronemia (C65Y, C65S, C68G, and Q115P). All four mutations involve highly conserved residues within GPIHBP1's Ly6 domain.This review will provide an update on GPIHBP1's role in the processing of chylomicrons and the pathogenesis of chylomicronemia.

Lipolysis Needed for Chylomicron Uptake?

Despite clinical evidence that postprandial lipemia and chylomicrons could contribute to atherosclerosis, direct evidence is lacking. The study by Weinstein et al(1) provides evidence to suggest that intact chylomicrons might be atherogenic by using genetically altered mice lacking Gpihbp1 protein, which may play a major role in the lipolysis of triglyceride-rich lipoproteins. However, the study does not rule out a contribution by remnants or limited lipolysis by other potential enzymes or pathways. It might be intriguing to determine the contribution of cholesterol derivatives in the chylomicrons to the lesions and the nature of the lesions.

Human Apolipoprotein A-II Determines Plasma Triglycerides by Regulating Lipoprotein Lipase Activity and High-Density Lipoprotein Proteome

Apolipoprotein (apo) A-II is the second most abundant high-density lipoprotein (HDL) apolipoprotein. We assessed the mechanism involved in the altered postprandial triglyceride-rich lipoprotein metabolism of female human apoA-II-transgenic mice (hapoA-II-Tg mice), which results in up to an 11-fold increase in plasma triglyceride concentration. The relationships between apoA-II, HDL composition, and lipoprotein lipase (LPL) activity were also analyzed in a group of normolipidemic women. Triglyceride-rich lipoprotein catabolism was decreased in hapoA-II-Tg mice compared to control mice. This suggests that hapoA-II, which was mainly associated with HDL during fasting and postprandially, impairs triglyceride-rich lipoprotein lipolysis. HDL isolated from hapoA-II-Tg mice impaired bovine LPL activity. Two-dimensional gel electrophoresis, mass spectrometry, and immunonephelometry identified a marked deficiency in the HDL content of apoA-I, apoC-III, and apoE in these mice. In normolipidemic women, apoA-II concentration was directly correlated with plasma triglyceride and inversely correlated with the HDL-apoC-II+apoE/apoC-III ratio [corrected]. HDL-mediated induction of LPL activity was inversely correlated with apoA-II and directly correlated with the HDL-apoC-II+apoE/apoC-III ratio [corrected]. Purified hapoA-II displaced apoC-II, apoC-III, and apoE from human HDL2. Human HDL3 was, compared to HDL2, enriched in apoA-II but poorer in apoC-II, apoC-III, and apoE. ApoA-II plays a crucial role in triglyceride catabolism by regulating LPL activity, at least in part, through HDL proteome modulation.

Inflammation associated with the postprandial lipolysis of triglyceriderich lipoproteins by lipoprotein lipase

Although hypertriglyceridemia has repeatedly been implicated as an atherogenic condition, there are conflicting reports concerning the atherogenicity of products released from triglyceride-rich lipoproteins by lipoprotein lipase. The hydrolysis of triglyceride is a normal process by which chylomicrons and very low-density lipoproteins are metabolized and cleared from the circulation, which would suggest a beneficial role for lipoprotein lipase in reducing circulating levels of triglyceride and, therefore, reducing atherosclerotic burden. However, many in vitro studies have shown that lipolysis products such as fatty acids induce vascular cell inflammation, which can initiate or exacerbate atherosclerosis. This review summarizes the results and implications of recent studies on the effects of lipoprotein lipase on vascular inflammation, expanding upon existing controversy among human postprandial studies, animal models, and in vitro experimental models.

Triglyceride-rich lipoprotein lipolysis releases neutral and oxidized FFAs that induce endothelial cell inflammation

Triglyceride-rich lipoprotein (TGRL) lipolysis products provide a pro-inflammatory stimulus that can alter endothelial barrier function. To probe the mechanism of this lipolysis-induced event, we evaluated the pro-inflammatory potential of lipid classes derived from human postprandial TGRL by lipoprotein lipase (LpL). Incubation of TGRL with LpL for 30 min increased the saturated and unsaturated FFA content of the incubation solutions significantly. Furthermore, concentrations of the hydroxylated linoleates 9-hydroxy ocatadecadienoic acid (9-HODE) and 13-HODE were elevated by LpL lipolysis, more than other measured oxylipids. The FFA fractions elicited pro-inflammatory responses inducing TNFalpha and intracellular adhesion molecule expression and reactive oxygen species (ROS) production in human aortic endothelial cells (HAECs). The FFA-mediated increase in ROS was blocked by both the cytochrome P450 2C9 inhibitor sulfaphenazole and NADPH oxidase inhibitors. Compared with linoleate, 13-HODE was found to be a more potent inducer of ROS production in HAECs, an activity that was insensitive to both NADPH oxidase and cytochrome P450 inhibitors. Therefore, although the oxidative metabolism of FFA in endothelial cells can produce inflammatory responses, TGRL lipolysis can also release preformed mediators of oxidative stress (e.g., HODEs) that may influence endothelial cell function in vivo by stimulating intracellular ROS production.

The Acidic Domain of GPIHBP1 Is Important for the Binding of Lipoprotein Lipase and Chylomicrons

GPIHBP1, a glycosylphosphatidylinositol-anchored endothelial cell protein of the lymphocyte antigen 6 (Ly6) family, plays a key role in the lipolysis of triglyceride-rich lipoproteins (e.g. chylomicrons). GPIHBP1 is expressed along the luminal surface of endothelial cells of heart, skeletal muscle, and adipose tissue, and GPIHBP1-expressing cells bind lipoprotein lipase (LPL) and chylomicrons avidly. GPIHBP1 contains an amino-terminal acidic domain (amino acids 24-48) that is enriched in aspartate and glutamate residues, and we previously speculated that this domain might be important in binding ligands. To explore the functional importance of the acidic domain, we tested the ability of polyaspartate or polyglutamate peptides to block the binding of ligands to pgsA-745 Chinese hamster ovary cells that overexpress GPIHBP1. Both polyaspartate and polyglutamate blocked LPL and chylomicron binding to GPIHBP1. Also, a rabbit antiserum against the acidic domain of GPIHBP1 blocked LPL and chylomicron binding to GPIHBP1-expressing cells. Replacing the acidic amino acids within GPIHBP1 residues 38-48 with alanine eliminated the ability of GPIHBP1 to bind LPL and chylomicrons. Finally, mutation of the positively charged heparin-binding domains within LPL and apolipoprotein AV abolished the ability of these proteins to bind to GPIHBP1. These studies indicate that the acidic domain of GPIHBP1 is important and that electrostatic interactions play a key role in ligand binding.

Apolipoprotein AV: gene expression,physiological role in lipid metabolism and clinical relevance

The apolipoprotein APOA5 gene, a member of the gene cluster on chromosome 11q23 that includes APOA1, APOC3 and APOA4, has gained considerable interest as it encodes ApoAV, a key determinant of circulating levels of potentially atherogenic triglyceride-rich lipoproteins (TRL). Indeed, strong associations between genetic variants of the APOA5 gene sequence and elevated triglyceride (TG) levels have been established. This apolipoprotein may potentiate lipolysis of TRL through facilitation of lipoprotein interaction with lipoprotein lipase. In addition, ApoAV may enhance clearance of remnant lipoproteins by mediating their interaction with the LDL receptor-related protein (LRP)1. The implication of ApoAV in intravascular TRL metabolism is further supported by studies that have demonstrated upregulation of APOA5 gene expression by nuclear receptors (PPARα, FXR and HNF4α) and hormones (thyroxine) involved in hypotriglyceridemic pathways. APOA4 expression may equally be modulated by nutritional status and, more ...

Triglyceride-rich lipoprotein lipolysis increases aggregation of endothelial cell membrane microdomains and produces reactive oxygen species

Triglyceride-rich lipoprotein (TGRL) lipolysis may provide a proinflammatory stimulus to endothelium. Detergent-resistant plasma membrane microdomains (lipid rafts) have a number of functions in endothelial cell inflammation. The mechanisms of TGRL lipolysis-induced endothelial cell injury were investigated by examining endothelial cell lipid rafts and production of reactive oxygen species (ROS). Lipid raft microdomains in human aortic endothelial cells were visualized by confocal microscopy with fluorescein isothiocyanate-labeled cholera toxin B as a lipid raft marker. Incubation of Atto565-labeled TGRL with lipid raft-labeled endothelial cells showed that TGRL colocalized with the lipid rafts, TGRL lipolysis caused clustering and aggregation of lipid rafts, and colocalization of TGRL remnant particles on the endothelial cells aggregated lipid rafts. Furthermore, TGRL lipolysis caused translocation of low-density lipoprotein receptor-related protein, endothelial nitric oxide synthase, and caveolin-1 from raft regions to nonraft regions of the membrane 3 h after treatment with TGRL lipolysis. TGRL lipolysis significantly increased the production of ROS in endothelial cells, and both NADPH oxidase and cytochrome P-450 inhibitors reduced production of ROS. Our studies suggest that alteration of lipid raft morphology and composition and ROS production could contribute to TGRL lipolysis-mediated endothelial cell injury.

Avian apolipoprotein A-V binds to LDL receptor gene family members

Apolipoprotein A-V (apoA-V) affects plasma triglyceride (TG) levels; however, the properties of apoA-V that mediate its action(s) are still incompletely understood. It is unclear how apoA-V, whose plasma concentration is extremely low, can affect the pronounced TG differences observed in individuals with various apoA-V dysfunctions. To gain novel insights into apoA-V biology, we expanded our previous studies in the chicken to this apolipoprotein. First, we characterized the first avian apoA-V, revealing its expression not only in liver and small intestine but also in brain, kidney, and ovarian follicles and showing its presence in the circulation. Second, we demonstrate directly that galline apoA-V binds to the major LDL receptor family member (LR) of the laying hen and that this interaction does not depend on the association of the apolipoprotein with lipid or lipoproteins. We propose that a direct interaction with LRs may represent a novel, additional mechanism for the modulation of TG levels by apoA-V.

Phospholipid Transfer Protein Augments Apoptosis in THP-1–Derived Macrophages Induced by Lipolyzed Hypertriglyceridemic Plasma

Lipolysis of triglyceride-rich lipoproteins (TGRLPs) generates phospholipid-rich surface remnants and induces cytotoxic effects in adjacent vascular cells. We hypothesized that by integrating surface remnants into HDL, phospholipid transfer protein (PLTP) alleviates cytotoxicity. To test this hypothesis and gain insight into cytotoxicity during the postprandial phase in vivo, we injected normo-TG and hyper-TG human volunteers after a standardized fat meal (postprandial sample) with heparin, thereby stimulating lipolysis (postprandial heparinized sample). Incubation of (primary) human macrophages and primary human endothelial cells with postprandial heparinized hyper-TG plasma induced pronounced cytotoxic effects that were dose dependent on the TG content of the sample. No such effects were seen with normo-TG and postprandial hyper-TG plasma. In vitro lipolysis of VLDL and chylomicrons indicated that both lipoprotein fractions can cause cytotoxicity. Interestingly, in experiments with THP-1-derived macrophages stably transfected with PLTP, PLTP substantially augmented both net phospholipid uptake and apoptotic cell death due to postprandial heparinized hyper-TG plasma. We observed that activation of caspase-3/7, poly-ADP-ribose polymerase, and enhanced bioactivity of acid sphingomyelinase may all contribute to this augmented apoptosis. Our data show that lipolysis of TGRLPs and their remodelling by PLTP interact to disturb cellular phospholipid flux and intracellular signaling processes, ultimately leading to apoptosis in human macrophages and endothelial cells.

Lipid Disorders in Diabetes Mellitus and Current Management

Lipid disorders are common in diabetes mellitus (DM), and play crucial roles in the development of diabetic cardiovascular complications. Diabetic dyslipidemia is characterized by hypertriglyceridemia, increased levels of very low density lipoproteins (VLDL), small dense low density lipoprotein (LDL), and decreased levels of high density lipoprotein (HDL)-cholesterol. The activity of lipoprotein lipase is reduced in diabetic patients, which attenuates the lipolysis of triglyceride-rich lipoproteins and the uptake of free fatty acids. The increased uptake of triglycerides in liver promotes the production of VLDL. Hypertriglyceridemia promotes the exchange of cholesteryl ester from HDL to VLDL or LDL for triglycerides. Obesity or nephropathy deteriorates the dyslipidemia in DM patients. The initial management of lipid disorders in diabetic patients without cardiovascular disease is lifestyle intervention and glucose control. The abnormalities in the metabolism of LDL or HDL in diabetic patients often require pharmacological intervention. Target of LDL-cholesterol (LDL-c) is more restrict in diabetic patients than in non-diabetic subjects. Treatment with HMG-CoA reductase inhibitors (statins) effectively reduces LDL-c and cardiac events, and that was associated with moderately increases in HDL-c. The combination of ezetimibe with a statin helps to achieve LDL-c target in patients with unsatisfactory cholesterol lowering by statin alone. Fibrates (PPAR-α agonists) or PPAR-γ agonists reduce the levels of triglycerides and moderately elevate HDL-c. PPAR-γ agonists also improve insulin sensitivity. Cholesteryl ester transfer protein inhibitors may dramatically increase HDL-c. Lipid management has been considered as an effective approach to reduce cardiovascular risk in diabetes.

Lipolysis products from triglyceride-rich lipoproteins increase endothelial permeability, perturb zonula occludens-1 and F-actin, and induce apoptosis

Products generated from lipoprotein lipase-mediated hydrolysis of triglyceride-rich lipoproteins (TGRL) are reported to increase endothelial layer permeability. We hypothesize that these increases in permeability result from the active rearrangement and dissolution of the junctional barrier in human aortic endothelial cells, as well as induction of the apoptotic cascade. Human aortic endothelial cells were treated with TGRL lipolysis products generated from coincubation of human TGRL plus lipoprotein lipase. Measurement of transendothelial electrical resistance demonstrated a time-dependent decrease in endothelial barrier function in response to TGRL lipolysis products. Immunofluorescent localization of zonula occludens-1 (ZO-1) showed radial rearrangement along cell borders after 1.5 h of treatment with lipolysis products. A concurrent redistribution of F-actin from the cell body to the cell margins was observed via rhodamine phalloidin staining. Immunofluorescent imaging for occludin and vascular endothelial cadherin showed that these proteins relocalize as well, although these changes are less prominent than for ZO-1. Western analysis of cells exposed to lipolysis products for 3 h revealed the fragmentation of ZO-1, a reduction in occludin, and no change of vascular endothelial cadherin. Lipolysis products also increased caspase-3 activity and induced nuclear fragmentation. Treatments did not cause oncosis in cells at any point during the incubation. These results demonstrate that TGRL lipolysis products play an important role in the regulation of endothelial permeability, the organization of the actin cytoskeleton, the localization and expression of junctional proteins, especially ZO-1, and the induction of apoptosis.