Anti-bis(monoacylglycero)phosphate antibody accumulates acetylated LDL-derived cholesterol in cultured macrophages

Amiodarone improves anemia in a murine model of sickle cell disease and is associated with increased erythrocyte bis(monoacylglycerol) phosphate

Sickle cell disease (SCD) is associated with altered plasma and erythrocyte lipid profiles. In a previous study, SCD mice with deficiency of proprotein convertase subtilisin/kexin type 9 (PCSK9) were observed to have more severe anemia and increased sickling compared to control SCD mice. Although PCSK9 affects circulating low density lipoprotein (LDL) by regulation of the LDL receptor, the effect of PCSK9 on anemia was independent of LDL receptor expression. In the current study, erythrocyte metabolomics were performed and revealed altered erythrocyte lipid species between SCD mice with and without PCSK9. Of particular interest, the late endosome-specific lipid bis(mono)acylglycerol phosphate (BMP) 44:12 was markedly decreased in erythrocytes from SCD mice deficient in PCSK9 mice relative to control SCD mice. Incubation of sickle erythrocytes with a neutralizing antibody to BMP increased erythrocyte sickling in vitro. In vitro treatment of SCD erythrocytes with amiodarone (1.5 μM) or medroxyprogesterone (6.75 μM), two pharmacologic compounds known to increase BMP, resulted in reduced erythrocyte sickling. Treatment of SCD mice with amiodarone (10 mg/kg) for 2 weeks resulted in increased BMP, improvement in anemia with reduced reticulocytosis, and decreased ex vivo sickling. In conclusion, severity of anemia in SCD is improved with amiodarone treatment, an effect which may be mediated through increased erythrocyte BMP.

Life in the lumen: The multivesicular endosome

The late endosomes/endo‐lysosomes of vertebrates contain an atypical phospholipid, lysobisphosphatidic acid (LBPA) (also termed bis[monoacylglycero]phosphate [BMP]), which is not detected elsewhere in the cell. LBPA is abundant in the membrane system present in the lumen of this compartment, including intralumenal vesicles (ILVs). In this review, the current knowledge on LBPA and LBPA‐containing membranes will be summarized, and their role in the control of endosomal cholesterol will be outlined. Some speculations will also be made on how this system may be overwhelmed in the cholesterol storage disorder Niemann‐Pick C. Then, the roles of intralumenal membranes in endo‐lysosomal dynamics and functions will be discussed in broader terms. Likewise, the mechanisms that drive the biogenesis of intralumenal membranes, including ESCRTs, will also be discussed, as well as their diverse composition and fate, including degradation in lysosomes and secretion as exosomes. This review will also discuss how intralumenal membranes are hijacked by pathogenic agents during intoxication and infection, and what is the biochemical composition and function of the intra‐endosomal lumenal milieu. Finally, this review will allude to the size limitations imposed on intralumenal vesicle functions and speculate on the possible role of LBPA as calcium chelator in the acidic calcium stores of endo‐lysosomes.

The endosomal lipid bis(monoacylglycero) phosphate as a potential key player in the mechanism of action of chloroquine against SARS-COV-2 and other enveloped viruses hijacking the endocytic pathway

The anti-malarial drug Chloroquine (CQ) and its derivative hydroxychloroquine have shown antiviral activities in vitro against many viruses, including coronaviruses, dengue virus and the biosafety level 4 Nipah and Hendra paramyxoviruses. The in vivo efficacy of CQ in the treatment of COVID-19 is currently a matter of debate. CQ is a lysosomotrophic compound that accumulates in lysosomes, as well as in food vacuoles of Plasmodium falciparum. In the treatment of malaria, CQ impairs the digestion and growth of the parasite by increasing the pH of the food vacuole. Similarly, it is assumed that the antiviral effects of CQ results from the increase of lysosome pH and the inhibition of acidic proteases involved in the maturation of virus fusion protein. CQ has however other effects, among which phospholipidosis, characterized by the accumulation of multivesicular bodies within the cell. The increase in phospholipid species particularly concerns bis(monoacylglycero)phosphate (BMP), a specific lipid of late endosomes involved in vesicular trafficking and pH-dependent vesicle budding. It was shown previously that drugs like progesterone, the cationic amphiphile U18666A and the phospholipase inhibitor methyl arachidonyl fluoro phosphonate (MAFP) induce the accumulation of BMP in THP-1 cells and decrease cell infection by human immunodeficiency virus. HIV viral particles were found to be retained into large endosomal-type vesicles, preventing virus spreading. Since BMP was also reported to favour virus entry through hijacking of the endocytic pathway, we propose here that BMP could play a dual role in viral infection, with its antiviral effects triggered by lysosomotropic drugs like CQ.

Bis(monoacylglycero)phosphate, an important actor in the host endocytic machinery hijacked by SARS-CoV-2 and related viruses

Viruses, including the novel coronavirus SARS-CoV-2, redirect infected cell metabolism to their own purposes. After binding to its receptor angiotensin-converting enzyme 2 (ACE2) on the cell surface, the SARS-CoV-2 is taken up by receptor-mediated endocytosis ending in the acidic endolysosomal compartment. The virus hijacks the endosomal machinery leading to fusion of viral and endosomal membranes and release of the viral RNA into the cytosol. This mini-review specifically highlights the membrane lipid organization of the endosomal system focusing on the unconventional and late endosome/lysosome-specific phospholipid, bis(monoacylglycero)phosphate (BMP). BMP is enriched in alveolar macrophages of lung, one of the target tissue of SARS-CoV-2. This review details the BMP structure, its unsaturated fatty acid composition and fusogenic properties that are essential for the highly dynamic formation of the intraluminal vesicles inside the endosomes. Interestingly, BMP is necessary for infection and replication of enveloped RNA virus such as SARS-CoV-1 and Dengue virus. We also emphasize the role of BMP in lipid sorting and degradation, especially cholesterol transport in cooperation with Niemann Pick type C proteins (NPC 1 and 2) and with some oxysterol-binding protein (OSBP)-related proteins (ORPs) as well as in sphingolipid degradation. Interestingly, numerous virus infection required NPC1 as well as ORPs along the endocytic pathway. Furthermore, BMP content is increased during pathological endosomal lipid accumulation in various lysosomal storage disorders. This is particularly important knowing the high percentage of patients with metabolic disorders among the SARS-CoV-2 infected patients presenting severe forms of COVID-19.

Intracellular cholesterol trafficking is dependent upon NPC2 interaction with lysobisphosphatidic acid

Unesterified cholesterol accumulation in the late endosomal/lysosomal (LE/LY) compartment is the cellular hallmark of Niemann-Pick C (NPC) disease, caused by defects in the genes encoding NPC1 or NPC2. We previously reported the dramatic stimulation of NPC2 cholesterol transport rates to and from model membranes by the LE/LY phospholipid lysobisphosphatidic acid (LBPA). It had been previously shown that enrichment of NPC1-deficient cells with LBPA results in cholesterol clearance. Here we demonstrate that LBPA enrichment in human NPC2-deficient cells, either directly or via its biosynthetic precursor phosphtidylglycerol (PG), is entirely ineffective, indicating an obligate functional interaction between NPC2 and LBPA in cholesterol trafficking. We further demonstrate that NPC2 interacts directly with LBPA and identify the NPC2 hydrophobic knob domain as the site of interaction. Together these studies reveal a heretofore unknown step of intracellular cholesterol trafficking which is critically dependent upon the interaction of LBPA with functional NPC2 protein.

Cholesterol is a type of fat that is essential for many processes in the body, such as repairing damaged cells and producing certain hormones. Normally, cholesterol enters cells from the bloodstream and is then moved to the parts of the cell that need it via a process known as ‘trafficking’.

When cholesterol trafficking goes wrong, abnormally large amounts of cholesterol and other fats accumulate within the cell. Over time, these fatty deposits become toxic to cells and eventually damage the affected tissues. Niemann-Pick type C disease (NPC) is a severe genetic disorder affecting cholesterol trafficking. It is characterized by cholesterol build-up in multiple tissues, including the brain, which ultimately causes degeneration and death of nerve cells.

Two proteins, NPC1 and NPC2, are involved in NPC disease. Both proteins normally help move cholesterol out of important trafficking compartments (known as the endosomal and lysosomal compartments) to other areas of the cell where it is needed. Patients with the disease can have mutations in either the gene for NPC1 or the gene for NPC2. This means that cells from NPC1 patients do not make enough functional NPC1 protein (but contain working NPC2), and vice versa.

Previous studies had shown that giving cells with NPC1 mutations large amounts of the small molecule lysobisphosphatidic acid (LBPA for short) could compensate for the loss of NPC1, and stop the toxic build-up of cholesterol. McCauliff, Langan, Li et al. therefore wanted to explore exactly how LBPA was doing this. They had shown that LBPA dramatically increased the ability of purified NPC2 protein to transport cholesterol, and wondered if the effect of LBPA in the cells without NPC1 depended on NPC2. They predicted that boosting LBPA levels would not work in cells lacking NPC2.

Biochemical experiments using purified protein showed that LBPA and NPC2 did indeed interact directly with each other. Systematically changing different building blocks of NPC2 revealed that a single region of the protein is sensitive to LBPA, and when this region was altered, LBPA could no longer interact with NPC2. Since LBPA is naturally produced by cells, they then stimulated cells grown in the laboratory to generate more LBPA using its precursor phosphatidylglycerol. They used cells from patients with mutations in either NPC1 or NPC2 and demonstrated that LBPA’s ability to reverse the accumulation of cholesterol was dependent on its interaction with NPC2. Thus, increasing LBPA levels in cells from patients with NPC1 mutations was beneficial, but had no effect on cells from patients with NPC2 mutations.

These results shed new light not only on how cells transport cholesterol, but also on potential methods to combat disorders of cellular cholesterol trafficking. In the future, LBPA could be developed as a genetically tailored, patient-specific therapy for diseases like NPC.

In vitro oxidized HDL and HDL from type 2 diabetes patients have reduced ability to efflux oxysterols from THP-1 macrophages

Oxidized LDL (OxLDL) that are enriched in products of lipid peroxidation including oxysterols have been shown to induce cellular oxidative stress and cytotoxicity therefore accelerating atheroma plaque formation. Upon oxLDL exposure of THP-1 macrophages, intracellular oxidation of LDL derived-cholesterol as well as endogenous cholesterol was increased. The oxysterols intracellularly produced were efficiently exported to HDL whereas apolipoprotein A1 was inefficient. These findings prompted us to investigate the consequences of modification of HDL by oxidation and glycation as observed in type 2 diabetes with respect to oxysterol and cholesterol efflux. We show that efflux of oxysterols was significantly impaired after in vitro oxidation and glycoxidation of HDL whereas glycation alone had no impact. Cholesterol efflux was only slightly decreased by oxHDL or glycoxidized HDL and not changed with glycated HDL. The defect of HDL towards oxysterol efflux was also observed with HDL isolated from diabetic subjects as compared to healthy controls. These findings support a deleterious cellular retention of oxysterols due to dysfunctional HDL in type 2 diabetes.

Bis(monoacylglycero)phosphate inhibits TLR4-dependent RANTES production in macrophages

Toll-like receptor 4 (TLR4) is the receptor for bacterial lipopolysaccharide (LPS) triggering production of pro-inflammatory cytokines which help eradicate the bacteria but could also be harmful when overproduced. The signaling activity of TLR4 is modulated by cholesterol level in cellular membranes, which in turn is affected by bis(monoacylglycero)phosphate (BMP), a phospholipid enriched in late endosomes. We found that exogenously added BMP isomers become incorporated into the plasma membrane and intracellular vesicles of macrophages and strongly reduced LPS-stimulated production of a chemokine RANTES, which was correlated with inhibition of interferon regulatory factor 3 (IRF3) controlling Rantes expression. To investigate the mechanism underlying the influence of BMP on TLR4 signaling we applied Laurdan and studied the impact of BMP incorporation on lipid packing, a measure for membrane order. Enrichment of model and cellular membranes with BMP significantly reduced their order and the reduction was maintained during stimulation of cells with LPS. This effect of BMP was abolished by enrichment of macrophages with cholesterol. In parallel, the inhibitory effect of BMP exerted on the TLR4-dependent phosphorylation of IRF3 was also reversed. Taken together our results indicate that BMP reduces the order of macrophage membranes which contributes to the inhibition of TLR4-dependent RANTES production.

A novel role for the Krüppel-like factor 14 on macrophage inflammatory response and atherosclerosis development

Genome-wide association studies have shown that Krüppel-like factor 14 (KLF14) is associated with both Type 2 diabetes mellitus and lipid metabolism. However, its role in chronic inflammatory responses and the pathogenesis of atherosclerosis remains unknown. The present study was designed to investigate both in vivo and in vitro the impact of KLF14 on chronic inflammatory responses and atherosclerosis. ApoE KO mice, a well-established animal model of atherosclerosis, had higher expressions of KLF14 in aorta tissues than that in C57BL/6 J mice when fed the high-fat diet (HFD) or standard chow diet. Adenovirus-mediated KLF14 knockdown markedly reduced the circulating levels of proinflammatory cytokines and the formation of atherosclerotic lesions in HFD-fed ApoE KO mice. In the in vitro study, KLF14 overexpression in the RAW264.7 macrophages significantly increased the expressions of inflammatory cytokines, total cholesterol (TC), cholesteryl ester (CE), and the ratio of CE to TC in the cells treated with acetylated low-density lipoproteins (AcLDL). Conversely, KLF14 knockdown remarkably attenuated AcLDL-induced increase in TC, CE, and the ratio of CE to TC as well as the expressions of inflammatory cytokines. Furthermore, up-regulation or down-regulation of KLF14 markedly elevated or inhibited the phosphorylation levels of p38 MAPK and ERK1/2 in AcLDL-stimulated RAW264.7 macrophages, respectively. Importantly, treatment with p38 MAPK or ERK1/2 inhibitor nullified the effects of KLF14 on inflammatory cytokine expressions in the cells. These data demonstrate an important role for KLF14 expression in atherosclerotic lesion formation.

Identification of the phospholipid lysobisphosphatidic acid in the protozoan Entamoeba histolytica: An active molecule in endocytosis

Phospholipids are essential for vesicle fusion and fission and both are fundamental events for Entamoeba histolytica phagocytosis. Our aim was to identify the lysobisphosphatidic acid (LBPA) in trophozoites and investigate its cellular fate during endocytosis. LBPA was detected by TLC in a 0.5 R_f spot of total lipids, which co-migrated with the LBPA standard. The 6C4 antibody, against LBPA recognized phospholipids extracted from this spot. Reverse phase LC-ESI-MS and MS/MS mass spectrometry revealed six LBPA species of m/z 772.58–802.68. LBPA was associated to pinosomes and phagosomes. Intriguingly, during pinocytosis, whole cell fluorescence quantification showed that LBPA dropped 84% after 15 min incubation with FITC-Dextran, and after 60 min, it increased at levels close to steady state conditions. Similarly, during erythrophagocytosis, after 15 min, LBPA also dropped in 36% and increased after 60 and 90 min. EhRab7A protein appeared in some vesicles with LBPA in steady state conditions, but after phagocytosis co-localization of both molecules increased and in late phases of erythrophagocytosis they were found in huge phagosomes or multivesicular bodies with many intraluminal vacuoles, and surrounding ingested erythrocytes and phagosomes. The 6C4 and anti-EhADH (EhADH is an ALIX family protein) antibodies and Lysotracker merged in about 50% of the vesicles in steady state conditions and throughout phagocytosis. LBPA and EhADH were also inside huge phagosomes. These results demonstrated that E. histolytica LBPA is associated to pinosomes and phagosomes during endocytosis and suggested differences of LBPA requirements during pinocytosis and phagocytosis.

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LBPA is identified for the first time in the protozoan Entamoeba histolytica.

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LBPA is found in pinosomes and in 10–20 µm diameter phagosomes or multivesicular bodies.

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LBPA appeared associated with EhRab7A protein, a late endosomes marker.

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LBPA interacts with EhADH (an ALIX family protein) during phagocytosis.

α/β Hydrolase Domain-containing 6 (ABHD6) Degrades the Late Endosomal/Lysosomal Lipid Bis(monoacylglycero)phosphate

α/β Hydrolase domain-containing 6 (ABHD6) can act as monoacylglycerol hydrolase and is believed to play a role in endocannabinoid signaling as well as in the pathogenesis of obesity and liver steatosis. However, the mechanistic link between gene function and disease is incompletely understood. Here we aimed to further characterize the role of ABHD6 in lipid metabolism. We show that mouse and human ABHD6 degrade bis(monoacylglycero)phosphate (BMP) with high specific activity. BMP, also known as lysobisphosphatidic acid, is enriched in late endosomes/lysosomes, where it plays a key role in the formation of intraluminal vesicles and in lipid sorting. Up to now, little has been known about the catabolism of this lipid. Our data demonstrate that ABHD6 is responsible for ∼90% of the BMP hydrolase activity detected in the liver and that knockdown of ABHD6 increases hepatic BMP levels. Tissue fractionation and live-cell imaging experiments revealed that ABHD6 co-localizes with late endosomes/lysosomes. The enzyme is active at cytosolic pH and lacks acid hydrolase activity, implying that it degrades BMP exported from acidic organelles or de novo-formed BMP. In conclusion, our data suggest that ABHD6 controls BMP catabolism and is therefore part of the late endosomal/lysosomal lipid-sorting machinery.

THP1 macrophages oxidized cholesterol, generating 7-derivative oxysterols specifically released by HDL

Macrophages are well recognized as key pathophysiologic agents in many chronic inflammatory diseases, especially atherosclerosis. During atherogenesis process, low density lipoproteins (LDL) undergo oxidation (oxLDL) and become highly atherogenic as they induce a strong accumulation of cholesterol in subendothelial macrophages leading to the formation of foam cells, the major cellular component of fatty streaks. OxLDL are enriched in oxidation products of cholesterol called oxysterols involved in the regulation of cholesterol homeostasis, by their ability to induce cellular oxidative stress and cytotoxicity. Little is known about intracellular oxysterol production in macrophages. Using both radiochemical and mass analyzes, we showed that THP1 macrophages promote the intracellular oxidation of LDL derived-cholesterol as well as intracellular cholesterol, this later mechanism being enhanced by exposure with native or oxLDL. We demonstrated that in both THP1 and Raw 267.4 cells cholesterol oxidation occurs in the late endosomal compartment. Most oxysterols were produced by non-enzymatic routes (7-ketocholesterol and 7α/β-hydroxycholesterol) but enzymatically formed 7α-, 27-hydroxycholesterol were also quantified. Incubation of THP1 macrophages with nLDL or oxLDL, induced a 2- and 100-fold increase in oxysterol production, respectively. Both oxysterols derived from LDL cholesterol and cellular cholesterol were readily exported to HDL whereas apoA1 was inefficient, showing that HDL plays a major role in the removal of excess oxysterols in THP1 macrophages.

Exosomes as new vesicular lipid transporters involved in cell-cell communication and various pathophysiologies

Exosomes are nanovesicles that have emerged as a new intercellular communication system between an intracellular compartment of a donor cell towards the periphery or an internal compartment of a recipient cell. The bioactivity of exosomes resides not only in their protein and RNA contents but also in their lipidic molecules. Exosomes display original lipids organized in a bilayer membrane and along with the lipid carriers such as fatty acid binding proteins that they contain, exosomes transport bioactive lipids. Exosomes can vectorize lipids such as eicosanoids, fatty acids, and cholesterol, and their lipid composition can be modified by in-vitro manipulation. They also contain lipid related enzymes so that they can constitute an autonomous unit of production of various bioactive lipids. Exosomes can circulate between proximal or distal cells and their fate can be regulated in part by lipidic molecules. Compared to their parental cells, exosomes are enriched in cholesterol and sphingomyelin and their accumulation in cells might modulate recipient cell homeostasis. Exosome release from cells appears to be a general biological process. They have been reported in all biological fluids from which they can be recovered and can be monitors of specific pathophysiological situations. Thus, the lipid content of circulating exosomes could be useful biomarkers of lipid related diseases. Since the first lipid analysis of exosomes ten years ago detailed knowledge of exosomal lipids has accumulated. The role of lipids in exosome fate and bioactivity and how they constitute an additional lipid transport system are considered in this review.

Bis(monoacylglycero)phosphate accumulation in macrophages induces intracellular cholesterol redistribution, attenuates liver-X receptor/ATP-Binding cassette transporter A1/ATP-binding cassette transporter G1 pathway, and impairs cholesterol efflux

OBJECTIVE

Endosomal signature phospholipid bis(monoacylglycero)phosphate (BMP) has been involved in the regulation of cellular cholesterol homeostasis. Accumulation of BMP is a hallmark of lipid storage disorders and was recently reported as a noticeable feature of oxidized low-density lipoprotein-laden macrophages. This study was designed to delineate the consequences of macrophage BMP accumulation on intracellular cholesterol distribution, metabolism, and efflux and to unravel the underlying molecular mechanisms.

APPROACH AND RESULTS

We have developed an experimental design to specifically increase BMP content in RAW 264.7 macrophages. After BMP accumulation, cell cholesterol distribution was markedly altered, despite no change in low-density lipoprotein uptake and hydrolysis, cholesterol esterification, or total cell cholesterol content. The expression of cholesterol-regulated genes sterol regulatory element-binding protein 2 and hydroxymethylglutaryl-coenzyme A reductase was decreased by 40%, indicative of an increase of endoplasmic reticulum-associated cholesterol. Cholesterol delivery to plasma membrane was reduced as evidenced by the 20% decrease of efflux by cyclodextrin. Functionally, BMP accumulation reduced cholesterol efflux to both apolipoprotein A1 and high-density lipoprotein by 40% and correlated with a 40% decrease in mRNA contents of ATP-binding cassette transporter A1, ATP-binding cassette transporter G1, and liver-X receptor α and β. Foam cell formation induced by oxidized low-density lipoprotein exposure was exacerbated in BMP-enriched cells.

CONCLUSIONS

The present work shows for the first time a strong functional link between BMP and cholesterol-regulating genes involved in both intracellular metabolism and efflux. We propose that accumulation of cellular BMP might contribute to the deregulation of cholesterol homeostasis in atheromatous macrophages.

Progesterone and a phospholipase inhibitor increase the endosomal bis(monoacylglycero)phosphate content and block HIV viral particle intercellular transmission

Progesterone, the cationic amphiphile U18666A and a phospholipase inhibitor (Methyl Arachidonyl Fluoro Phosphonate, MAFP) inhibited by 70%-90% HIV production in viral reservoir cells, i.e. human THP-1 monocytes and monocyte-derived macrophages (MDM). These compounds triggered an inhibition of fluid phase endocytosis (macropinocytosis) and modified cellular lipid homeostasis since endosomes accumulated filipin-stained sterols and Bis(Monoacylglycero)Phosphate (BMP). BMP was quantified using a new cytometry procedure and was increased by 1.25 times with MAFP, 1.7 times with U18666A and 2.5 times with progesterone. MAFP but not progesterone or U18666A inhibited the hydrolysis of BMP by the Pancreatic Lipase Related Protein 2 (PLRP2) as shown by in-vitro experiments. The possible role of sterol transporters in steroid-mediated BMP increase is discussed. Electron microscopy showed the accumulation of viral particles either into large intracellular viral-containing compartments or outside the cells, indicating that endosomal accumulation of BMP could block intracellular biogenesis of viral particles while inhibition of macropinocytosis would prevent viral particle uptake. This is the first report linking BMP metabolism with a natural steroid such as progesterone or with involvement of a phospholipase A1 activity. BMP cellular content could be used as a biomarker for efficient anti-viral drugs.

Bis(monoacylglycero)phosphate reduces oxysterol formation and apoptosis in macrophages exposed to oxidized LDL

Atherosclerosis is a major cardiovascular complication of diseases associated with increased oxidative stress that favors oxidation of circulating low density lipoproteins (LDLs). Oxidized LDL (oxLDL) is considered as highly atherogenic as it induces a strong accumulation of cholesterol in subendothelial macrophages leading to the formation of foam cells and emergence of atherosclerotic plaque. OxLDL is enriched in oxidation products of cholesterol called oxysterols, some of which have been involved in the ability of oxLDL to induce cellular oxidative stress and cytotoxicity, mainly by apoptosis. Little is known about the possible contribution of cell-generated oxysterols toward LDL-associated oxysterols in cellular accumulation of oxysterols and related apoptosis. Using both radiochemical and mass analyzes, we showed that oxLDL greatly enhanced oxysterol production by RAW macrophages in comparison with unloaded cells or cells loaded with native LDL. Most oxysterols were produced by non-enzymatic routes (7-ketocholesterol and 7α/β-hydroyxycholesterol) but enzymatically formed 7α-, 25- and 27-hydroxycholesterol were also quantified. Bis(monoacylglycero)phosphate (BMP) is a unique phospholipid preferentially found in late endosomes. We and others have highlighted the role of BMP in the regulation of intracellular cholesterol metabolism/traffic in macrophages. We here report that cellular BMP accumulation was associated with a significantly lower production of oxysterols upon oxLDL exposure. Of note, potent pro-apoptotic 7-ketocholesterol was the most markedly decreased. OxLDL-induced cell cytotoxicity and apoptosis were consistently attenuated in BMP-enriched cells. Taken together, our data suggest that BMP exerts a protective action against the pro-apoptotic effect of oxLDL via a reduced production of intracellular pro-apoptotic oxysterols.

Effects of the endosomal lipid bis(monoacylglycero)phosphate on the thermotropic properties of DPPC: A 2H NMR and spin label EPR study

Bis(monoacylglycero)phosphate (BMP) is an endosomal lipid with a unique structure that is implicated in the formation of intraendosomal vesicular bodies. Here we have characterized the effects of dioleoyl-BMP (BMP(18:1)) at concentrations of 5, 10, 15 and 20mol% on the thermotropic behavior of dipalmitoyl phosphatidylcholine (DPPC) vesicles, and compared them to those of equimolar concentrations of dioleoyl phosphatidylglycerol (DOPG), a structural isoform of BMP(18:1). Because BMP is found in the acidic environments of the late endosome and intralysosomal vesicles, samples were prepared at pH 4.2 to mimic the pH of the lysosome. Both (2)H NMR of perdeuterated DPPC and spin-labeled EPR with 16-doxyl phosphatidylcholine were utilized in these investigations. NMR and EPR results show that BMP(18:1) induces a lowering in the main phase transition temperature of DPPC similar to that of DOPG. The EPR studies reveal that BMP(18:1) induced more disorder in the L(beta) phase when compared to equimolar concentrations of DOPG. Analysis from dePaked (2)H NMR spectra in the L(alpha) phase reveals that BMP(18:1) induces less disorder than equal concentrations of DOPG. Additionally, the results demonstrate that BMP mixes with other phospholipids as a phospholipid and not as a detergent molecule as once speculated.

Pdro, a protein associated with late endosomes and lysosomes and implicated in cellular cholesterol homeostasis

BACKGROUND

Cellular cholesterol is a vital component of the cell membrane. Its concentration is tightly controlled by mechanisms that remain only partially characterized. In this study, we describe a late endosome/lysosomes-associated protein whose expression level affects cellular free cholesterol content.

METHODOLOGY/PRINCIPAL FINDINGS

Using a restricted proteomic analysis of detergent-resistant membranes (DRMs), we have identified a protein encoded by gene C11orf59. It is mainly localized to late endosome/lysosome (LE/LY) compartment through N-terminal myristoylation and palmitoylation. We named it Pdro for protein associated with DRMs and endosomes. Very recently, three studies have reported on the same protein under two other names: the human p27RF-Rho that regulates RhoA activation and actin dynamics, and its rodent orthologue p18 that controls both LE/LY dynamics through the MERK-ERK pathway and the lysosomal activation of mammalian target of rapamycin complex 1 by amino acids. We found that, consistent with the presence of sterol-responsive element consensus sequences in the promoter region of C11orf59, Pdro mRNA and protein expression levels are regulated positively by cellular cholesterol depletion and negatively by cellular cholesterol loading. Conversely, Pdro is involved in the regulation of cholesterol homeostasis, since its depletion by siRNA increases cellular free cholesterol content that is accompanied by an increased cholesterol efflux from cells. On the other hand, cells stably overexpressing Pdro display reduced cellular free cholesterol content. Pdro depletion-mediated excess cholesterol results, at least in part, from a stimulated low-density lipoprotein (LDL) uptake and an increased cholesterol egress from LE/LY.

CONCLUSIONS/SIGNIFICANCE

LDL-derived cholesterol release involves LE/LY motility that is linked to actin dynamics. Because Pdro regulates these two processes, we propose that modulation of Pdro expression in response to sterol levels regulates LDL-derived cholesterol through both LDL uptake and LE/LY dynamics, to ultimately control free cholesterol homeostasis.

Bis(monoacylglycero)phosphate, a peculiar phospholipid to control the fate of cholesterol: Implications in pathology

Bis(monoacylglycero)phosphate (BMP) is a structural isomer of phosphatidylglycerol that exhibits an unusual sn1:sn1' stereoconfiguration, based on the position of the phosphate moiety on its two glycerol units. Early works have underlined the high concentration of BMP in the lysosomal compartment, especially during some lysosomal storage disorders and drug-induced phospholipidosis. Despite numerous studies, both biosynthetic and degradative pathways of BMP remained not completely elucidated. More recently, BMP has been localized in the internal membranes of late endosomes where it forms specialized lipid domains. Its involvement in both dynamics and lipid/protein sorting functions of late endosomes has started to be documented, especially in the control of cellular cholesterol distribution. BMP also plays an important role in the late endosomal/lysosomal degradative pathway. Another peculiarity of BMP is to be naturally enriched in docosahexaenoic acid and/or to specifically incorporate this fatty acid compared to other polyunsaturated fatty acids, which may confer specific biophysical and functional properties to this phospholipid. This review summarizes and updates our knowledge on BMP with an emphasis on its possible implication in human health and diseases, especially in relation to cholesterol homeostasis.

Cholesterol synthesis inhibitor U18666A and the role of sterol metabolism and trafficking in numerous pathophysiological processes

The multiple actions of U18666A have enabled major discoveries in lipid research and contributed to understanding the pathophysiology of multiple diseases. This review describes these advances and the utility of U18666A as a tool in lipid research. Harry Rudney's recognition that U18666A inhibited oxidosqualene cyclase led him to discover a pathway for formation of polar sterols that he proved to be important regulators of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase. Laura Liscum's recognition that U18666A inhibited the egress of cholesterol from late endosomes and lysosomes led to greatly improved perspective on the major pathways of intracellular cholesterol trafficking. The inhibition of cholesterol trafficking by U18666A mimicked the loss of functional Niemann-Pick type C protein responsible for NPC disease and thus provided a model for this disorder. U18666A subsequently became a tool for assessing the importance of molecular trafficking through the lysosomal pathway in other conditions such as atherosclerosis, Alzheimer's disease, and prion infections. U18666A also provided animal models for two important disorders: petite mal (absence) epilepsy and cataracts. This was the first chronic model of absence epilepsy. U18666A is also being used to address the role of oxidative stress in apoptosis. How can one molecule have so many effects? Perhaps because of its structure as an amphipathic cationic amine it can interact and inhibit diverse proteins. Restricting the availability of cholesterol for membrane formation through inhibition of cholesterol synthesis and intracellular trafficking could also be a mechanism for broadly affecting many processes. Another possibility is that through intercalation into membrane U18666A can alter membrane order and therefore the function of resident proteins. The similarity of the effects of natural and enantiomeric U18666A on cells and the capacity of intercalated U18666A to increase membrane order are arguments in favor of this possibility.

Cholesterol regulation of rab-mediated sphingolipid endocytosis

Despite a tight regulation of its intracellular content, cholesterol is found accumulated in pathological conditions such as sphingolipidosis as well as after cell treatment with drugs like hydrophobic amines. Furthermore, cellular cholesterol increases when cultured cells approach confluence. Under these conditions, the endocytic pathways of plasma membrane sphingolipids are differently affected. In this short review, we will summarize recent results from our laboratory as well as those of other groups, indicating that the intracellular accumulation of cholesterol inhibits the dissociation of rab GTPases from the target membranes, causing the alteration of rab-mediated membrane traffic.

Selective decrease of bis(monoacylglycero)phosphate content in macrophages by high supplementation with docosahexaenoic acid

Bis(monoacylglycero)phosphate (BMP) is a unique phospholipid (PL) preferentially found in late endosomal membranes, where it forms specialized lipid domains. Recently, using cultured macrophages treated with anti-BMP antibody, we showed that BMP-rich domains are involved in cholesterol homeostasis. We had previously stressed the high propensity of BMP to accumulate docosahexaenoic acid (DHA), compared with other PUFAs. Because phosphatidylglycerol (PG) was reported as a precursor for BMP synthesis in RAW macrophages, we examined the effects of PG supplementation on both FA composition and amount of BMP in this cell line. Supplementation with dioleoyl-PG (18:1/18:1-PG) induced BMP accumulation, together with an increase of oleate proportion. Supplementation with high concentrations of didocosahexaenoyl-PG (22:6/22:6-PG) led to a marked enrichment of DHA in BMP, resulting in the formation of diDHA molecular species. However, the amount of BMP was selectively decreased. Similar effects were observed after supplementation with high concentrations of nonesterified DHA. Addition of vitamin E prevented the decrease of BMP and further increased its DHA content. Supplementation with 22:6/22:6-PG promoted BMP accumulation with an enhanced proportion of 22:6/22:6-BMP. DHA-rich BMP was significantly degraded after cell exposure to oxidant conditions, in contrast to oleic acid-rich BMP, which was not affected. Using a cell-free system, we showed that 22:6/22:6-BMP is highly oxidizable and partially protects cholesterol oxidation, compared with 18:1/18:1-BMP. Our data suggest that high DHA content in BMP led to specific degradation of this PL, possibly through the diDHA molecular species, which is very prone to peroxidation and, as such, a potential antioxidant in its immediate vicinity.

De novo biosynthesis of the late endosome lipid, bis(monoacylglycero)phosphate

Bis(monoacylglycero)phosphate (BMP) is a unique lipid enriched in the late endosomes participating in the trafficking of lipids and proteins through this organelle. The de novo biosynthesis of BMP has not been clearly demonstrated. We investigated whether phosphatidylglycerol (PG) and cardiolipin (CL) could serve as precursors of de novo BMP synthesis using two different cellular models: CHO cells deficient in phosphatidylglycerophosphate (PGP) synthase, the enzyme responsible for the first step of PG synthesis; and human lymphoblasts from patients with Barth syndrome (BTHS), characterized by mutations in tafazzin, an enzyme implicated in the deacylation-reacylation cycle of CL. The biosynthesis of both PG and BMP was reduced significantly in the PGP synthase-deficient CHO mutants. Furthermore, overexpression of PGP synthase in the deficient mutants induced an increase of BMP biosynthesis. In contrast to CHO mutants, BMP biosynthesis and its fatty acid composition were not altered in BTHS lymphoblasts. Our results thus suggest that in mammalian cells, PG, but not CL, is a precursor of the de novo biosynthesis of BMP. Despite the decrease of de novo synthesis, the cellular content of BMP remained unchanged in CHO mutants, suggesting that other pathway(s) than de novo biosynthesis are also used for BMP synthesis.