Triglyceride-rich Lipoproteins Lipolysis Products Research Articles

Mitochondrial oxidative stress-induced transcript variants of ATF3 mediate lipotoxic brain microvascular injury

Elevation of blood triglycerides, primarily triglyceride-rich lipoproteins (TGRL), is an independent risk factor for cardiovascular disease and vascular dementia (VaD). Accumulating evidence indicates that both atherosclerosis and VaD are linked to vascular inflammation. However, the role of TGRL in vascular inflammation, which increases risk for VaD, remains largely unknown and its underlying mechanisms are still unclear. We strived to determine the effects of postprandial TGRL exposure on brain microvascular endothelial cells, the potential risk factor of vascular inflammation, resulting in VaD. We showed in Aung et al., J Lipid Res., 2016 that postprandial TGRL lipolysis products (TL) activate mitochondrial reactive oxygen species (ROS) and increase the expression of the stress-responsive protein, activating transcription factor 3 (ATF3), which injures human brain microvascular endothelial cells (HBMECs) in vitro. In this study, we deployed high-throughput sequencing (HTS)-based RNA sequencing methods and mito stress and glycolytic rate assays with an Agilent Seahorse XF analyzer and profiled the differential expression of transcripts, constructed signaling pathways, and measured mitochondrial respiration, ATP production, proton leak, and glycolysis of HBMECs treated with TL. Conclusions: TL potentiate ROS by mitochondria which activate mitochondrial oxidative stress, decrease ATP production, increase mitochondrial proton leak and glycolysis rate, and mitochondria DNA damage. Additionally, CPT1A1 siRNA knockdown suppresses oxidative stress and prevents mitochondrial dysfunction and vascular inflammation in TL treated HBMECs. TL activates ATF3-MAPKinase, TNF, and NRF2 signaling pathways. Furthermore, the NRF2 signaling pathway which is upstream of the ATF3-MAPKinase signaling pathway, is also regulated by the mitochondrial oxidative stress. We are the first to report differential inflammatory characteristics of transcript variants 4 (ATF3-T4) and 5 (ATF3-T5) of the stress responsive gene ATF3 in HBMECs induced by postprandial TL. Specifically, our data indicates that ATF3-T4 predominantly regulates the TL-induced brain microvascular inflammation and TNF signaling. Both siRNAs of ATF3-T4 and ATF3-T5 suppress cells apoptosis and lipotoxic brain microvascular endothelial cells. These novel signaling pathways triggered by oxidative stress-responsive transcript variants, ATF3-T4 and ATF3-T5, in the brain microvascular inflammation induced by TGRL lipolysis products may contribute to pathophysiological processes of vascular dementia.

Evaluation of free testosterone measurement on the ids-isys automated platform as compared to RIA

Elevation of blood triglycerides, primarily triglyceride-rich lipoproteins (TGRL), is an independent risk factor for cardiovascular disease and vascular dementia (VaD). Accumulating evidence indicates that both atherosclerosis and VaD are linked to vascular inflammation. However, the role of TGRL in vascular inflammation, which increases risk for VaD, remains largely unknown and its underlying mechanisms are still unclear. We strived to determine the effects of postprandial TGRL exposure on brain microvascular endothelial cells, the potential risk factor of vascular inflammation, resulting in VaD. We showed in Aung et al., J Lipid Res., 2016 that postprandial TGRL lipolysis products (TL) activate mitochondrial reactive oxygen species (ROS) and increase the expression of the stress-responsive protein, activating transcription factor 3 (ATF3), which injures human brain microvascular endothelial cells (HBMECs) in vitro. In this study, we deployed high-throughput sequencing (HTS)-based RNA sequencing methods and mito stress and glycolytic rate assays with an Agilent Seahorse XF analyzer and profiled the differential expression of transcripts, constructed signaling pathways, and measured mitochondrial respiration, ATP production, proton leak, and glycolysis of HBMECs treated with TL. Conclusions: TL potentiate ROS by mitochondria which activate mitochondrial oxidative stress, decrease ATP production, increase mitochondrial proton leak and glycolysis rate, and mitochondria DNA damage. Additionally, CPT1A1 siRNA knockdown suppresses oxidative stress and prevents mitochondrial dysfunction and vascular inflammation in TL treated HBMECs. TL activates ATF3-MAPKinase, TNF, and NRF2 signaling pathways. Furthermore, the NRF2 signaling pathway which is upstream of the ATF3-MAPKinase signaling pathway, is also regulated by the mitochondrial oxidative stress. We are the first to report differential inflammatory characteristics of transcript variants 4 (ATF3-T4) and 5 (ATF3-T5) of the stress responsive gene ATF3 in HBMECs induced by postprandial TL. Specifically, our data indicates that ATF3-T4 predominantly regulates the TL-induced brain microvascular inflammation and TNF signaling. Both siRNAs of ATF3-T4 and ATF3-T5 suppress cells apoptosis and lipotoxic brain microvascular endothelial cells. These novel signaling pathways triggered by oxidative stress-responsive transcript variants, ATF3-T4 and ATF3-T5, in the brain microvascular inflammation induced by TGRL lipolysis products may contribute to pathophysiological processes of vascular dementia.

P 198 - Role of ATF3 in mediating lipid-induced stress signaling in brain microvascular endothelial cells

Elevation of blood triglycerides, primarily triglyceride-rich lipoproteins (TGRL), is an independent risk factor for atherosclerotic cardiovascular disease and Vascular Dementia (VaD). The accumulating evidence indicates that the development of atherosclerosis and VaD are linked to vascular inflammation. Previous studies suggest a relationship of blood lipids and/or lipoproteins to vascular inflammation leading to dementia. The objective of this study is to characterize human brain microvascular endothelial cells (HBMVEC) responses to postprandial TGRL and determine the pathways that have potential to increase risk for vascular dementia. To address this question, we treated HBMVEC with TGRL lipolysis products and then we used NGS-based RNA-Seq methods for transcriptional profiling experiments. Our data demonstrated up-regulation of activating transcription factor 3 (ATF3)-dependent inflammatory pathways in HBMVEC. Additionally, system biology analysis exhibited putative lipid-related targets and corresponding isoforms of ATF3 that may be implicated in hypertriglyceridemia-induced vascular inflammation in brain. Furthermore, RNA-Seq analysis determined specific isoforms of key transcriptional regulator ATF3 and stress response signaling pathway in lipolysis products-mediated brain vascular inflammation which may lead to vascular dementia.

Triglyceride-rich lipoprotein lipolysis products increase blood-brain barrier transfer coefficient and induce astrocyte lipid droplets and cell stress.

Elevation of blood triglycerides, primarily as triglyceride-rich lipoproteins (TGRL), has been linked to cerebrovascular inflammation, vascular dementia, and Alzheimer's disease (AD). Brain microvascular endothelial cells and astrocytes, two cell components of the neurovascular unit, participate in controlling blood-brain barrier (BBB) permeability and regulating neurovascular unit homeostasis. Our studies showed that infusion of high physiological concentrations of TGRL lipolysis products (TGRL + lipoprotein lipase) activate and injure brain endothelial cells and transiently increase the BBB transfer coefficient (Ki = permeability × surface area/volume) in vivo. However, little is known about how blood lipids affect astrocyte lipid accumulation and inflammation. To address this, we first demonstrated TGRL lipolysis products increased lipid droplet formation in cultured normal human astrocytes. We then evaluated the transcriptional pathways activated in astrocytes by TGRL lipolysis products and found upregulated stress and inflammatory-related genes including activating transcription factor 3 (ATF3), macrophage inflammatory protein-3α (MIP-3α), growth differentiation factor-15 (GDF15), and prostaglandin-endoperoxide synthase 2 (COX2). TGRL lipolysis products also activated the JNK/cJUN/ATF3 pathway, induced endoplasmic reticulum stress protein C/EBP homologous protein (CHOP), and the NF-κB pathway, while increasing secretion of MIP-3α, GDF15, and IL-8. Thus our results demonstrate TGRL lipolysis products increase the BBB transfer coefficient (Ki), induce astrocyte lipid droplet formation, activate cell stress pathways, and induce secretion of inflammatory cytokines. Our observations are consistent with evidence for lipid-induced neurovascular injury and inflammation, and we, therefore, speculate that lipid-induced astrocyte injury could play a role in cognitive decline.

TGRL Lipolysis Products Induce Stress Protein ATF3 via the TGF-β Receptor Pathway in Human Aortic Endothelial Cells.

Studies have suggested a link between the transforming growth factor beta 1 (TGF-β1) signaling cascade and the stress-inducible activating transcription factor 3 (ATF3). We have demonstrated that triglyceride-rich lipoproteins (TGRL) lipolysis products activate MAP kinase stress associated JNK/c-Jun pathways resulting in up-regulation of ATF3, pro-inflammatory genes and induction of apoptosis in human aortic endothelial cells. Here we demonstrate increased release of active TGF-β at 15 min, phosphorylation of Smad2 and translocation of co-Smad4 from cytosol to nucleus after a 1.5 h treatment with lipolysis products. Activation and translocation of Smad2 and 4 was blocked by addition of SB431542 (10 μM), a specific inhibitor of TGF-β-activin receptor ALKs 4, 5, 7. Both ALK receptor inhibition and anti TGF-β1 antibody prevented lipolysis product induced up-regulation of ATF3 mRNA and protein. ALK inhibition prevented lipolysis product-induced nuclear accumulation of ATF3. ALKs 4, 5, 7 inhibition also prevented phosphorylation of c-Jun and TGRL lipolysis product-induced p53 and caspase-3 protein expression. These findings demonstrate that TGRL lipolysis products cause release of active TGF-β and lipolysis product-induced apoptosis is dependent on TGF-β signaling. Furthermore, signaling through the stress associated JNK/c-Jun pathway is dependent on TGF-β signaling suggesting that TGF-β signaling is necessary for nuclear accumulation of the ATF3/cJun transcription complex and induction of pro-inflammatory responses.

Characterizing blood-brain barrier perturbations after exposure to human triglyceride-rich lipoprotein lipolysis products using MRI in a rat model.

PurposePrevious studies indicated hyperlipidemia may be a risk factor for Alzheimer's disease, but the contributions of postprandial triglyceride‐rich lipoprotein (TGRL) are not known. In this study, changes in blood–brain barrier diffusional transport following exposure to human TGRL lipolysis products were studied using MRI in a rat model.MethodsMale Sprague‐Dawley rats (∼180–250 g) received an i.v. injection of lipoprotein lipase (LpL)‐hydrolyzed TGRL (n = 8, plasma concentration ≈ 150 mg human TGRL/dL). Controls received i.v. injection of either saline (n = 6) or LpL only (n = 6). The 1H longitudinal relaxation rate R1 = 1/T1 was measured over 18 min using a rapid‐acquired refocus‐echo (RARE) sequence after each of three injections of the contrast agent Gd‐DTPA. Patlak plots were generated for each pixel yielding blood‐to‐brain transfer coefficients, Ki, chosen for best fit to impermeable, uni‐directional influx or bi‐directional flux models using the F‐test.ResultsAnalysis from a 2‐mm slice, 2‐mm rostral to the bregma showed a 275% increase of mean Ki during the first 20 min after infusion of human TGRL lipolysis product that differed significantly compared with saline and LpL controls. This difference disappeared by 40 min mark.ConclusionThese results suggest human TGRL lipolysis products can lead to a transient increase in rat BBB permeability. Magn Reson Med 76:1246–1251, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

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 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.

Dual role of lipoproteins in endothelial cell dysfunction in atherosclerosis

The endothelium is a key constituent of the vascular wall, being actively involved in maintaining the structural integrity and proper functioning of blood vessels. Hyperlipidemia, diabetes, hypertension, smoking and aging are important risk factors for the dysfunction of endothelial cells (EC). Circulating lipoproteins (Lp) synthesized and secreted from the intestine or liver have an important role in supplying peripheral tissues with fatty acids from triglyceride rich lipoproteins (TGRLp) for energy production or storage, and cholesterol from low density lipoproteins (LDL) or high density lipoproteins (HDL) for the synthesis of cellular membranes and steroid hormones. Under pathological conditions, Lp may suffer alterations in concentration and composition and become aggressors for EC. Modified LDL, remnant Lp, TGRLp lipolysis products, dysfunctional HDL are involved in the changes induced in EC morphology (reduced glycocalyx, overdeveloped endoplasmic reticulum, Golgi apparatus and basement membrane), loose intercellular junctions, increased oxidative and inflammatory stress, nitric oxide/redox imbalance, excess Lp transport and storage, as well as loss of anti-thrombotic properties, all of these being characteristics of endothelial dysfunction. Normal HDL are able to counteract the harmful effects of atherogenic Lp in EC but under persistent pathological conditions they lose the protective properties and become pro-atherogenic. This review summarises recent advances in understanding the role of Lp in the induction of endothelial dysfunction and the initiation and progression of atherosclerotic lesions. Its main focus is the antagonistic role of atherogenic Lp (LDL, VLDL, dysfunctional HDL) versus anti-atherogenic Lp (HDL), also pointing out the potential targets for arresting or reversing this process.

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

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.

Lipolysis products from triglyceride‐rich lipoproteins induce endothelial cell apoptosis by stimulating TGF‐beta1 release and Smad signaling

Several studies have demonstrated that a wide variety of stimuli can induce programmed cell death (apoptosis) in endothelial cells, and have suggested that apoptosis could be an important mechanism in the development of atherosclerosis. TGF-β1 has been shown to induce apoptosis in endothelial cells by activating members of the Smad family. Lipolysis products generated from lipoprotein lipase (LpL)-mediated hydrolysis of triglyceride-rich lipoproteins (TGRL) are recently described as independent risk factors for the development of atherosclerosis. We hypothesize that TGRL lipolysis products can induce endothelial cell apoptosis by stimulating the release of TGF-β1, initiating Smad signaling in human aortic endothelial cells (HAEC). To test this hypothesis we treated HAECs with lipolysis products generated from co-incubation of 150 mg/dL of TGRL plus 2 units/mL of LpL. Analysis of cell supernatants by ELISA showed a rise in TGF-β1 release from cells treated with lipolysis products compared to controls. Western Blotting showed a significant increase in expression of phospho-Smad2 after 1 hour of treatment with lipolysis products. Immunofluorescent staining for Smad4 demonstrated significant translocation of Smad4 to the nucleus after 1 hour. Fluorimetric analysis revealed significant activation of caspase-3 after 2 hours of treatment. Together these findings reveal for the first time a possible signaling mechanism by which lipolysis products can induce apoptosis in HAECs. This work was supported by the Nora Eccles Treadwell Foundation and NIH HL 55667.

Lipolysis products alter protein and lipid structural characteristics in the plasma membranes of human monocytes

The interactions between lipids and monocytes are fundamental to the development of atherosclerosis. Postprandial lipemia is characterized by increased circulating triglyceride-rich lipoproteins (TGRLs). Prolonged exposure of these lipids and their lipolysis products to cellular membranes is thought to promote atherosclerosis. We hypothesize that TGRL lipolysis products activate monocytes, generating novel cellular phenotypes that predict atherosclerosis. We investigated this hypothesis by treating primary human monocytes with either TGRL lipolysis products (150 mg/dL TG + 3 U lipoprotein lipase) or media for 3.5 hours. Fourier transform infrared radiation (FTIR) spectra of the amide signature region were obtained from treated and untreated cells, and their metabolic signatures determined by direct nano-electrospray ionization FT-MS. FTIR spectra of TGRL-treated cells revealed a spectral signature characterized by proteins with increased β-sheet conformations, possibly due to changes in inflammatory marker expression. FT-MS analysis showed an increase in phosphatidylcholine and desaturases in TGRL-treated cells, indicating changes in membrane fluidity. These results demonstrate that lipid lipolysis products exert drastic changes on monocyte surfaces and may contribute to the development of monocyte signatures that could predict atherosclerosis. This research was partially supported by a fellowship under the Training Program in Biomolecular Technology (T32-GM08799) at the University of California, Davis.

Endothelial progenitor cell repair in cardiovascular disease

Endothelial progenitor cells (EPC) are continuously present in blood and have reparative functions for injured endothelial cells and smooth muscle cells in both arteries and the heart. Previous studies show that triglyceride-rich lipoproteins (TGRL), and especially lipolysis products of TGRL, can injure and modify endothelial cells, monocytes, and platelets. This suggests that lipids and lipoproteins normally present in blood could affect circulating EPC levels and EPC function. The overall goals of this project are to develop the technological platforms to nondestructively characterize, image, and sort individual EPCs. We hypothesize that high concentrations of triglyceride-rich lipoprotein (TGRL) lipolysis products injure EPCs and impair functionality. Lipid raft-labeled EPCs were visualized by confocal laser scanning microscopy to observe morphological changes induced by TGRL and/or its lipolysis products over time. Fourier Transform Infrared Radiation (FTIR) spectroscopy on EPCs treated with TGRL lipolysis products shows severe degradation and/or aggregation of protein secondary structures. Currently underway are functional assays for viability, proliferation, migration, senescence, and apoptosis. These results indicate that lipids normally present in blood may damage circulating EPCs and possibly impair their ability to repair damaged endothelium. This work is supported by University of California at Davis Stem Cell Seed grant, NIH HL 78615 minority supplement, and NSF Center for Biophotonic Sciences and Technology.