Degradation Of Low Density Lipoprotein Research Articles

Brain Endothelial Cells in Contrary to the Aortic Do Not Transport but Degrade Low-Density Lipoproteins via Both LDLR and ALK1.

The transport of low-density lipoprotein (LDL) through the endothelium is a key step in the development of atherosclerosis, but it is notorious that phenotypic differences exist between endothelial cells originating from different vascular beds. Endothelial cells forming the blood–brain barrier restrict paracellular and transcellular passage of plasma proteins. Here, we systematically compared brain versus aortic endothelial cells towards their interaction with LDL and the role of proteins known to regulate the uptake of LDL by endothelial cells. Both brain endothelial cells and aortic endothelial cells bind and internalize LDL. However, whereas aortic endothelial cells degrade very small amounts of LDL and transcytose the majority, brain endothelial cells degrade but do not transport LDL. Using RNA interference (siRNA), we found that the LDLR–clathrin pathway leads to LDL degradation in either endothelial cell type. Both loss- and gain-of-function experiments showed that ALK1, which promotes transcellular LDL transport in aortic endothelial cells, also limits LDL degradation in brain endothelial cells. SR-BI and caveolin-1, which promote LDL uptake and transport into aortic endothelial cells, limit neither binding nor association of LDL to brain endothelial cells. Together, these results indicate distinct LDL trafficking by brain microvascular endothelial cells and aortic endothelial cells.

Pterostilbene Increases LDL Metabolism in HL-1 Cardiomyocytes by Modulating the PCSK9/HNF1α/SREBP2/LDLR Signaling Cascade, Upregulating Epigenetic hsa-miR-335 and hsa-miR-6825, and LDL Receptor Expression.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) can promote the degradation of low-density lipoprotein (LDL) receptor (LDLR), leading to hypercholesterolemia and myocardial dysfunction. The intracellular regulatory mechanism by which the natural polyphenol pterostilbene modulates the PCSK9/LDLR signaling pathway in cardiomyocytes has not been evaluated. We conducted Western blotting, flow cytometry, immunofluorescence staining, and mean fluorescence intensity analyses of pterostilbene-treated mouse HL-1 cardiomyocytes. Pterostilbene did not alter cardiomyocyte viability. Compared to the control group, treatment with both 2.5 and 5 μM pterostilbene significantly increased the LDLR protein expression accompanied by increased uptake of LDL. The expression of the mature PCSK9 was significantly suppressed at the protein and mRNA level by the treatment with both 2.5 and 5 μM pterostilbene, respectively, compared to the control. Furthermore, 2.5 and 5 μM pterostilbene treatment resulted in a significant reduction in the protein hepatic nuclear factor 1α (HNF1α)/histone deacetylase 2 (HDAC2) ratio and sterol regulatory element-binding protein-2 (SREBP2)/HDAC2 ratio. The expression of both hypoxia-inducible factor-1 α (HIF1α) and nuclear factor erythroid 2-related factor 2 (Nrf2) at the protein level was also suppressed. Pterostilbene as compared to short hairpin RNA against SREBP2 induced a higher protein expression of LDLR and lower nuclear accumulation of HNF1α and SREBP2. In addition, pterostilbene reduced PCSK9/SREBP2 interaction and mRNA expression by increasing the expression of hsa-miR-335 and hsa-miR-6825, which, in turn, increased LDLR mRNA expression. In cardiomyocytes, pterostilbene dose-dependently decreases and increases the protein and mRNA expression of PCSK9 and LDLR, respectively, by suppressing four transcription factors, HNF1α, SREBP2, HIF1α, and Nrf2, and enhancing the expression of hsa-miR-335 and hsa-miR-6825, which suppress PCSK9/SREBP2 interaction.

New groups of hypolipidemic medications based on inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9). Part 1

Hypolipidemic therapy is one of the essential components for the management of patients with cardiovascular diseases (CVD). In this regard, the main task of modern research is to find new targets for creating additional effective groups of hypolipidemic medications. Canadian and French research groups led by N. Seidah and M. Abifadel discovered a new enzyme — proprotein convertase subtilisin-kexin type 9 (PCSK9) in 2003. It turned out to play an important role in lipid metabolism later. The main mechanism of action of PCSK9 is to regulate the density of low-density lipoprotein receptors (LDLR) in the cell membrane of hepatocytes. Increased activity of PCSK9 accelerates the degradation of LDL significantly, and leads to an increase in the concentration of atherogenic classes of lipoproteins — low-density lipoproteins (LDL). In contrast, reduced PCSK9 activity is accompanied by a decrease in LDL concentrations and a reduced risk of developing atherosclerosis and CVD. The second of the recently discovered and less studied mechanism of PCSK9 protearogenic action is an increase in inflammatory processes in the atherosclerotic plaque. Considering this adverse contribution of PCSK9 to the development and progression of atherosclerosis and CVD, the main task of the researchers was to develop medications that inhibit THIS enzyme. Several new groups of medications that target the stages of biosynthesis and the function of PCSK9 have been developed by now. In this article, we will focus on details discussing the mechanisms of action and effectiveness of the following groups of hypolipidemic medications: anti-PCSK9 monoclonal antibodies (alirocumab, evolocumab), small interfering ribonucleic acids (incliciran), and antisense nucleotides.

SUMOylation of the ubiquitin ligase IDOL decreases LDL receptor levels and is reversed by SENP1

Inducible degrader of the low-density lipoprotein receptor (IDOL) is an E3 ubiquitin ligase mediating degradation of low-density lipoprotein (LDL) receptor (LDLR). IDOL also controls its own stability through autoubiquitination, primarily at lysine 293. Whether IDOL may undergo other forms of posttranslational modification is unknown. In this study, we show that IDOL can be modified by small ubiquitin-like modifier 1 at the K293 residue at least. The SUMOylation of IDOL counteracts its ubiquitination and augments IDOL protein levels. SUMOylation and the associated increase of IDOL protein are effectively reversed by SUMO-specific peptidase 1 (SENP1) in an activity-dependent manner. We further demonstrate that SENP1 affects LDLR protein levels by modulating IDOL. Overexpression of SENP1 increases LDLR protein levels and enhances LDL uptake in cultured cells. On the contrary, loss of SENP1 lowers LDLR levels in an IDOL-dependent manner and reduces LDL endocytosis. Collectively, our results reveal SUMOylation as a new regulatory posttranslational modification of IDOL and suggest that SENP1 positively regulates the LDLR pathway via deSUMOylation of IDOL and may therefore be exploited for the treatment of cardiovascular disease.

The α-defensins stimulate proteoglycan-dependent catabolism of low-density lipoprotein by vascular cells: a new class of inflammatory apolipoprotein and a possible contributor to atherogenesis

AbstractInflammation may contribute to the pathogenesis of atherosclerosis. On the basis of previous reports that human atherosclerotic lesions contain α-defensins, a class of cationic proteins released by activated neutrophils, the study was designed to ask whether defensins modulate the binding and catabolism of low-density lipoprotein (LDL) by human vascular cells. The results of the study demonstrated that defensin stimulated the binding of 125I-LDL to cultured human umbilical vein endothelial cells, smooth muscle cells, and fibroblasts approximately 5-fold in a dose-dependent and saturable manner. Defensin and LDL formed stable complexes in solution and on cell surfaces. Stimulation of LDL binding by defensin was not inhibited by antibodies against the LDL-receptor (LDL-R), or by recombinant receptor-associated protein, which blocks binding of ligands to the α2-macroglobulin receptor/LDL-R–related protein and other LDL-R family members. Furthermore, defensin stimulated the binding, endocytosis, and degradation of LDL by fibroblasts lacking LDL-R. Stimulation of LDL degradation by defensin was inhibited approximately 75% by low concentrations of heparin (0.2 units/mL) and was similarly reduced in CHO cells lacking heparan-sulfate–containing proteoglycans. The effect of defensin was substantially increased in cells overexpressing the core protein of the syndecan-1 heparan sulfate proteoglycan. The α-defensins released from activated neutrophils may provide a link between inflammation and atherosclerosis by changing the pattern of LDL catabolism from LDL-R to the less efficient LDL-R–independent, proteoglycan-dependent pathway.

Proprotein convertase subtilisin/kexin type 9 inhibition in cardiovascular disease: current status and future perspectives.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) targets the degradation of low-density lipoprotein (LDL) receptors; it has been proved that its inhibition improves cardiovascular outcomes in patients with established atherosclerotic cardiovascular disease (ASCVD). Herein, we review the current status of PCSK9 inhibitors in clinical practice and the future scope of PCSK9 inhibition. The results of two recent large clinical trials reveal that two PCSK9 monoclonal antibodies evolocumab and alirocumab reduce the risk of a cardiovascular event on top of background statin therapy in patients with stable ASCVD and those with recent acute coronary syndrome, respectively. However, there are several ongoing concerns regarding the efficacy in reducing mortality, cost-effectiveness, and long-term safety of extremely low LDL cholesterol levels with PCSK9 inhibition. The results of ongoing cardiovascular outcomes trials with PCSK9 monoclonal antibodies for primary prevention and with small interfering RNA to PCSK9 for secondary prevention may help to shape the use of this new therapeutic class.

PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9): A Narrative Review

Coronary artery disease (CAD), the most common type of heart disease, is the leading reason for mortality in both developing and developed countries. Increased cholesterol and fatty deposits (called plaques) may cause hardening or narrowing of the arteries which supply blood to heart muscles. Triglycerides, low density lipoproteins (LDL), high density lipoproteins (HDL) are different types of cholesterols found in the blood and LDL is the main target for lipid modifying therapy, with the aim of improving long term CAD prognosis. Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) is one of the candidate genes with expression of PCSK9 protein and PCSK9 inhibitors are coming up as newer lipid lowering therapy. This narrative review highlights the journey of drug development from recognition of PCSK9 gene to the recent approvals of PCSK9 targeting LDL lowering pharmacotherapy. A bibliographic survey was made with titles PCSK9, PCSK9 inhibitors and coronary artery disease in different search engines from year 2000 to 2019 and filtered with review, preclinical and clinical studies. Retrieved articles were revisited and it was observed that PCSK9 is expressed mainly in hepatocytes and to some extent in mesenchymal cells of kidney, intestinal ileum, colon epithelia and in telencephalon neurons of embryonic brain. In hepatocytes, loss-of function mutations of PCSK9 leads to higher levels of LDL receptors. These receptors make LDL receptor-LDL cholesterol complex, which is directed to the lysosome for degradation of LDL in hepatocytes and lowers LDL cholesterol levels, ultimately resulting in protection from CAD. Gain-of-function mutations hamper LDL degradation. PCSK9 research has proposed an exciting new area for cholesterol management and CAD risk reduction. Different PCSK9 inhibitors with different therapeutic targets for CAD are evolving day by day from bench to bedside. This review could be valuable for helping researchers acquire a deeper insight for PCSK9 and its Inhibitors.

Interruption of Autophagosome Formation in Cardiovascular Disease, an Evidence for Protective Response of Autophagy

ABSTRACTBackground: A heart attack occurs when coronary artery blockage interrupts the blood supply to the heart such as is seen in cardiovascular disease (CVD). Importantly, autophagy is commonly regarded as a host defense mechanism against microbial invaders. Methods: A total of 50 blood samples were obtained from cardiovascular (CV) patients in addition to 30 samples that were obtained from healthy individuals and served as controls. Macrophages were isolated in vitro and propagated from the blood samples. Autophagosome formation, cytokine secretion, and apolipoprotein B (ApoB) gene expression were monitored in patient samples and their derived macrophages. Results: The results showed that autophagy-related (Atg) LC3 and Atg5 genes were significantly down-regulated in all samples obtained from CV patients. Furthermore, the relative gene expression of ApoB, which plays the major role in lipoprotein metabolism, was significantly increased in CV patients. Interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) levels were increased in these blood samples. Interestingly, targeting of ApoB by small interference RNA (siRNA) reduced the production levels of low-density lipoprotein (LDL), IL-6 and TNF-α in patient-derived macrophages. Further, treatment of patient-derived macrophages with rapamycin, an autophagy inducer agent, successfully regulated the production of LDL, IL-6, TNF-α, and ApoB expression via activation of autophagosome formation. Conclusion: The current data reveal the potential disturbance of autophagy in CV patients that accompanied ApoB over-expression. Furthermore, our findings provide evidence for the protective role of autophagy in accumulation of pro-inflammatory cytokines and intracellular LDL degradation in CV patient-derived macrophages.

The Major Molecular Causes of Familial Hypercholesterolemia

Familial Hypercholesterolemia (FH) is a common dominant disorder of cholesterol metabolism characterized by elevated serum cholesterol level which results in increasing risk of many diseases. The major cause of FH is the loss-of-function in Low Density Lipoprotein Receptor (LDLR), Apolipoprotein B-100 (ApoB-100), Low Density Lipoprotein Receptor Adapter Protein (LDLRAP1), and Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9) gene that revealed to the defects in the uptake and degradation of Low Density Lipoprotein (LDL) via the LDLR pathway. In this review, we have highlighted the molecular disorder in LDLR, ApoB-100, LDLRAP1 and PCSK gene, leading to the possible accession on early diagnosis, screening of FH based on the clinical characteristics, family history, evaluated LDL-Cholesterol levels and recently genetic testing aided, hence molecular based therapy will be applied or recommended to FH patients.

Acute Myelogenous Leukemia Cells Secrete Factors that Stimulate Cellular LDL Uptake via Autocrine and Paracrine Mechanisms

Leukemic cells isolated from most patients with acute myelogenous leukemia (AML) have higher low density lipoprotein (LDL) uptake than normal mononuclear blood cells. Little is known, however, about the mechanism behind the elevated LDL uptake. We investigated if AML cells secrete factors that stimulate cellular LDL uptake. Mononuclear blood cells were isolated from peripheral blood from 42 patients with AML at diagnosis. Cellular LDL uptake was determined from the degradation rate of 125I-labelled LDL. Conditioned media from AML cells stimulated the LDL degradation in the leukemic cell lines KG1 and HL60, and in isolated AML cells. The stimulatory effect correlated with the LDL degradation in the AML cells directly after isolation from blood. Conditioned media also autostimulated LDL degradation in the AML cells themselves. Concentrations of IL-6 and IL-8 in AML cell conditioned media correlated with the LDL degradation in AML cells directly after isolation from blood. Addition of R-TNF-α, but not IL-6 or IL-8, stimulated LDL degradation in HL60, KG1, and AML cells. The LDL degradation in AML cells could be inhibited by a LDL receptor blocking antibody. AML cells secrete factors that stimulate LDL uptake in a paracrine and autocrine pattern which open up therapeutic possibilities to inhibit the uptake of LDL by administration of antibodies to these factors.

The Proprotein Convertase Subtilisin/Kexin Type 9-resistant R410S Low Density Lipoprotein Receptor Mutation: A NOVEL MECHANISM CAUSING FAMILIAL HYPERCHOLESTEROLEMIA

Familial hypercholesterolemia (FH) is characterized by severely elevated low density lipoprotein (LDL) cholesterol. Herein, we identified an FH patient presenting novel compound heterozygote mutations R410S and G592E of the LDL receptor (LDLR). The patient responded modestly to maximum rosuvastatin plus ezetimibe therapy, even in combination with a PCSK9 monoclonal antibody injection. Using cell biology and molecular dynamics simulations, we aimed to define the underlying mechanism(s) by which these LDLR mutations affect LDL metabolism and lead to hypercholesterolemia. Our data showed that the LDLR-G592E is a class 2b mutant, because it mostly failed to exit the endoplasmic reticulum and was degraded. Even though LDLR-R410S and LDLR-WT were similar in levels of cell surface and total receptor and bound equally well to LDL or extracellular PCSK9, the LDLR-R410S was resistant to exogenous PCSK9-mediated degradation in endosomes/lysosomes and showed reduced LDL internalization and degradation relative to LDLR-WT. Evidence is provided for a tighter association of LDL with LDLR-R410S at acidic pH, a reduced LDL delivery to late endosomes/lysosomes, and an increased release in the medium of the bound/internalized LDL, as compared with LDLR-WT. These data suggested that LDLR-R410S recycles loaded with its LDL-cargo. Our findings demonstrate that LDLR-R410S represents an LDLR loss-of-function through a novel class 8 FH-causing mechanism, thereby rationalizing the observed phenotype.

Advances in the field of proprotein convertase subtilisin kexin type 9 inhibitors.

Proprotein convertase subtilisin kexin type 9 (PCSK9) inhibitors are promising therapies that inhibit the degradation of low-density lipoprotein (LDL) receptors in the hepatocyte and thus increase LDL cholesterol (LDL-C) uptake from the blood. This review summarizes main findings in the field of PCSK9 inhibitors, from basic mechanism to clinical studies, and aims to provide a contemporary and practical overview of the clinical implication and future directions with PCSK9 inhibitors. Monoclonal antibodies that inhibit PCSK9 reduce LDL-C levels by 40-70% across a wide range of patients with various LDL-C levels, and with different lipid-lowering regimens. These agents significantly reduce apolipoprotein B and lipoprotein (a), may have a potential role in plaque stabilization in acute coronary syndromes, and are safe and tolerable, even among statin-intolerant patients. Preliminary data with evolocumab and alirocumab demonstrate the potential reduction of cardiovascular (CV) events. These PCSK9 inhibitors were recently approved for clinical use, and recommended in the 2016 American College of Cardiology expert consensus document for nonstatin therapy for LDL-C lowering. PCSK9 inhibitors are novel promising therapies to reduce LDL-C. Ongoing phase 3 clinical trials with more than 70 000 high-risk patients will examine their safety and efficacy in reducing cardiovascular disease.

Altered metabolism of LDL in the arterial wall precedes atherosclerosis regression.

Plasma cholesterol lowering is beneficial in patients with atherosclerosis. However, it is unknown how it affects entry and degradation of low-density lipoprotein (LDL) particles in the lesioned arterial wall. We studied the effect of lipid-lowering therapy on LDL permeability and degradation of LDL particles in atherosclerotic aortas of mice by measuring the accumulation of iodinated LDL particles in the arterial wall. Cholesterol-fed, LDL receptor-deficient mice were treated with either an anti-Apob antisense oligonucleotide or a mismatch control antisense oligonucleotide once a week for 1 or 4 weeks before injection with preparations of iodinated LDL particles. The anti-Apob antisense oligonucleotide reduced plasma cholesterol by ≈90%. The aortic LDL permeability and degradation rates of newly entered LDL particles were reduced by ≈50% and ≈85% already after 1 week of treatment despite an unchanged pool size of aortic iodinated LDL particles. In contrast, the size, foam cell content, and aortic pool size of iodinated LDL particles of aortic atherosclerotic plaques were not reduced until after 4 weeks of treatment with the anti-Apob antisense oligonucleotide. Improved endothelial barrier function toward the entry of plasma LDL particles and diminished aortic degradation of the newly entered LDL particles precede plaque regression.

Abstract 122: Ceramide Activation of Macrophage RhoA/Rho Kinase/LIM Kinase Signaling Impairs Aggregated LDL Degradation and Foam Cell Formation

Introduction: One of the initiating events in atherogenesis is the deposition of low-density lipoproteins (LDL) in the arterial wall. This LDL becomes modified, aggregated and retained. Macrophages form a degradative contact (a lysosomal synapse or LS) with the aggregated LDL (agLDL) through local actin polymerization. Degradation of agLDL releases free cholesterol, which is taken up by macrophages and results in foam cell formation. Characterization of proteins regulating actin polymerization that allows LS formation may identify new therapeutic targets to inhibit foam cell formation and halt the progression of atherosclerosis. Hypothesis: Small GTPases of the Rho family such as RhoA are key regulators of actin polymerization. Further, many lipids in the micro-environment of the atherosclerotic plaque, such as ceramide, are known to modulate RhoA activity. Therefore we tested the hypothesis that RhoA is important in actin polymerization and foam cell formation in response to macrophage catabolism of agLDL and that this process can be modulated by ceramide. Methods: We overexpressed RhoA in RAW264.7 macrophages prior to treatment with agLDL to test the role of RhoA in LS formation and foam cell formation. We used bone marrow derived macrophages (BMM) from Sphingosine Kinase 2 knockout mice (SK2 KO) as a model for macrophages loaded with long chain ceramides and wild-type BMM loaded with C2-ceramide to test the role of ceramide. We also used inhibitors of Rho Kinase ROCK and LIM Kinase to assess their role in LS formation. Results and Conclusions: Overexpression of wild-type and constitutive active RhoA in RAW264.7 macrophages significantly impaired actin polymerization and foam cell formation in response to agLDL treatment. Using SK2KO BMM and wild-type BMM loaded with C2- ceramide, we find that macrophages loaded with ceramide display increased levels of activated RhoA. This leads to impaired actin polymerization and foam cell formation which could be rescued by RNAi mediated silencing of RhoA, inhibition of ROCK or inhibition of LIMK. In conclusion, these results suggest that RhoA/ROCK/LIMK signaling negatively regulates agLDL degradation and foam cell formation and that this process can be directly modulated by ceramide through activation of RhoA.

An Update on the Clinical Development of Proprotein Convertase Subtilisin Kexin 9 Inhibitors, Novel Therapeutic Agents for Lowering Low‐Density Lipoprotein Cholesterol

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays an essential role in the degradation of low-density lipoprotein C (LDL-C) receptors, and PCSK9 inhibitors have recently emerged as a potential treatment option to reduce LDL-C. Our paper reviewed the current available Phase II clinical trials of PCSK9 inhibitors for the treatment of dyslipidemia. A second objective of this review was to evaluate the potential clinical role of PCSK9 inhibitors in the management of dyslipidemia. Studies evaluating the efficacy and safety of any PCSK9 inhibitors in patients with dyslipidemia were included. The monoclonal antibodies REGN727/SAR236553 and AMG145 have the most published clinical data. Seven phase II trials were retrieved that evaluated the efficacy and safety of REGN727/SAR236553 or AMG145 in patients with either hypercholesterolemia or heterozygous familial hypercholesterolemia (HeFH). These two agents significantly decreased LDL-C levels either as monotherapy or in combination with other lipid-lowering agents. REGN727/SAR236553 and AMG145 have been well tolerated. The ongoing phase III trials of these two agents are summarized. REGN727/SAR236553 and AMG145 have demonstrated the potential to further decrease LDL-C levels when added to conventional lipid-lowering therapy. Morbidity and mortality data are required to define their roles in clinical practice.

159 Computational approach for optimizing inhibitors of 1HWK (HMG-CoA reductase)

Coronary heart disease is a leading cause of death, and represents an increasing burden on healthcare resources worldwide. It is one of the most investigated diseases in Medicinal Chemistry. HMG-CoA reductase (3-hydroxy-3-methyl-glutaryl-CoA reductase or HMGCR) is the rate-controlling enzyme of the mevalonate pathway, the metabolic pathway that produces cholesterol and other isoprenoids (Istvan et al., 2001). Normally in mammalian cells this enzyme is suppressed by cholesterol derived from the internalization and degradation of low density lipoprotein (LDL) via LDL receptor as well as oxidized species of cholesterol. This enzyme is thus the target of the widely available cholesterol-lowering drugs collectively known as statins (Da Silva et al., 2008). In the present study, crystal structure of HMG-coA 1HWK was prepared. Active site was identified. A virtual screening was performed (Schrödinger, LLC, New York, NY) against ligand data-set prepared from different literature search and ZINC (Irwin et al., 2012) database to identify the lead molecules. These lead molecules were optimized using molecular dynamics and show good docking score than statins.

Cholesteryl ester acyl oxidation and remodeling in murine macrophages: formation of oxidized phosphatidylcholine

Cholesterol is an essential component of eukaryotic cell membranes, regulating fluidity and permeability of the bilayer. Outside the membrane, cholesterol is esterified to fatty acids forming cholesterol esters (CEs). Metabolism of CEs is characterized by recurrent hydrolysis and esterification as part of the CE cycle; however, since recombinant 15-lipoxygenase (15-LO) was shown to oxidize cholesteryl linoleate of LDL, there has been interest in CE oxidation, particularly in the context atherogenesis. Studies of oxidized CE (oxCE) metabolism have focused on hydrolysis and subsequent reverse cholesterol transport with little emphasis on the fate the newly released oxidized fatty acyl component. Here, using mass spectrometry to analyze lipid oxidation products, CE metabolism in murine peritoneal macrophages was investigated. Ex vivo macrophage incubations revealed that cellular 15-LO directly oxidized multiple CE substrates from intracellular stores and from extracellular sources. Freshly harvested murine macrophages also contained 15-LO-specific oxCEs, suggesting the enzyme may act as a CE-oxidase in vivo. The metabolic fate of oxCEs, particularly the hydrolysis and remodeling of oxidized fatty acyl chains, was also examined in the macrophage. Metabolism of deuterated CE resulted in the genesis of deuterated, oxidized phosphatidylcholine (oxPC). Further experiments revealed these oxPC species were formed chiefly from the hydrolysis of oxidized CE and subsequent reacylation of the oxidized acyl components into PC.

Low Density Lipoprotein Receptor-Related Protein 1 Dependent Endosomal Trapping and Recycling of Apolipoprotein E

BackgroundLipoprotein receptors from the low density lipoprotein (LDL) receptor family are multifunctional membrane proteins which can efficiently mediate endocytosis and thereby facilitate lipoprotein clearance from the plasma. The biggest member of this family, the LDL receptor-related protein 1 (LRP1), facilitates the hepatic uptake of triglyceride-rich lipoproteins (TRL) via interaction with apolipoprotein E (apoE). In contrast to the classical LDL degradation pathway, TRL disintegrate in peripheral endosomes, and core lipids and apoB are targeted along the endocytic pathway for lysosomal degradation. Notably, TRL-derived apoE remains within recycling endosomes and is then mobilized by high density lipoproteins (HDL) for re-secretion. The aim of this study is to investigate the involvement of LRP1 in the regulation of apoE recycling.Principal FindingsImmunofluorescence studies indicate the LRP1-dependent trapping of apoE in EEA1-positive endosomes in human hepatoma cells. This processing is distinct from other LRP1 ligands such as RAP which is efficiently targeted to lysosomal compartments. Upon stimulation of HDL-induced recycling, apoE is released from LRP1-positive endosomes but is targeted to another, distinct population of early endosomes that contain HDL, but not LRP1. For subsequent analysis of the recycling capacity, we expressed the full-length human LRP1 and used an RNA interference approach to manipulate the expression levels of LRP1. In support of LRP1 determining the intracellular fate of apoE, overexpression of LRP1 significantly stimulated HDL-induced apoE recycling. Vice versa LRP1 knockdown in HEK293 cells and primary hepatocytes strongly reduced the efficiency of HDL to stimulate apoE secretion.ConclusionWe conclude that LRP1 enables apoE to accumulate in an early endosomal recycling compartment that serves as a pool for the intracellular formation and subsequent re-secretion of apoE-enriched HDL particles.

Proprotein Convertase Substilisin/Kexin Type 9 Antagonism Reduces Low-Density Lipoprotein Cholesterol in Statin-Treated Hypercholesterolemic Nonhuman Primates

Proprotein convertase substilisin/kexin type 9 (PCSK9) promotes the degradation of low-density lipoprotein (LDL) receptor (LDLR) and thereby increases serum LDL-cholesterol (LDL-C). We have developed a humanized monoclonal antibody that recognizes the LDLR binding domain of PCSK9. This antibody, J16, and its precursor mouse antibody, J10, potently inhibit PCSK9 binding to the LDLR extracellular domain and PCSK9-mediated down-regulation of LDLR in vitro. In vivo, J10 effectively reduces serum cholesterol in C57BL/6 mice fed normal chow. J16 reduces LDL-C in healthy and diet-induced hypercholesterolemic cynomologous monkeys, but does not significantly affect high-density lipoprotein-cholesterol. Furthermore, J16 greatly lowered LDL-C in hypercholesterolemic monkeys treated with the HMG-CoA reductase inhibitor simvastatin. Our data demonstrate that anti-PCSK9 antibody is a promising LDL-C-lowering agent that is both efficacious and potentially additive to current therapies.

Investigation of Lysosomes as Enzyme Storage Vesicles using Single Particle Tracking Fluorescence Microscopy

Intracellular, vesicle-mediated, degradation of extracellular cargo is an essential cellular function. Of particular interest is the population of vesicles responsible for degradation of extracellular cargo. Previous work using low-density lipoprotein (LDL), a classic extracellular cargo, demonstrates that the enzyme-mediated degradation of LDL occurs in a hybrid late endosomal-lysosomal vesiclei. In addition, the degradation of LDL occurs within 60 s of the colocalization of LDL with a lysosomal protein, LAMP1. These observations suggest that lysosomes are responsible for delivering enzymes to late endosomes, forming a hybrid organelle in which degradation occurs. Using single particle tracking fluorescence microscopy, we investigate the hypothesis that lysosomes serve as enzyme storage vesicles. We test this hypothesis using two different approaches. The first approach uses cell-permeable drugs that inhibit specific enzymes known to degrade LDL; CA074ME, a cathepsin B inhibitor, and pepstatin methyl ester, a cathepsin D inhibitor. The second approach uses siRNA to knockdown expression of LAMP. Unique to these experiments is the ability to directly monitor, in intact live cells, LDL degradation utilizing a fluorescence labeling scheme that reports on the integrity of the LDL particle. This method involves conjugation of distinct fluorophores to the protein and lipid components of LDL. Upon degradation the protein reporter decreases in fluorescence intensity while the lipid reporter increases fluorescence intensity. Our experiments make it possible to describe the complete endocytic pathway of LDL from internalization to degradation and provide a more complete picture of the intracellular degradation of extracellular cargo. i W.H. Humphries IV, N.C. Fay, and C.K. Payne, Intracellular degradation of low-density lipoprotein probed with two-color fluorescence microscopy, Integrative Biology, in press, (2010).