Metabolic fate of oleic acid derived from lysosomal degradation of cholesteryl oleate in human fibroblasts

Effects of 6-O-α-maltosyl-β cyclodextrin on lipid metabolism in Npc1-deficient Chinese hamster ovary cells

Niemann-Pick disease Type C (NPC) is a lysosomal storage disorder caused by mutation of the NPC1/NPC2 genes, which ultimately results in the accumulation of unesterified cholesterol (UEC) in lysosomes, thereby inducing symptoms such as progressive neurodegeneration and hepatosplenomegaly. This study determines the effects of 6-O-α-maltosyl-β cyclodextrin (Mal-βCD) on lipid levels and synthesis in Npc1-deficient (Npc1-KO cells) and vehicle CHO cells. Compared to vehicle cells, Npc1-KO cells exhibited high level of UEC, and low levels of esterified cholesterols (ECs) and long-chain fatty acids (LCFAs). The difference in lipid levels between Npc1-KO and CHO cells was largely ameliorated by Mal-βCD administration. Moreover, the effects of Mal-βCD were reproduced in the lysosomes prepared from Npc1-KO cells. Stable isotope tracer analysis with extracellular addition of D4-deuterated palmitic acid (D4-PA) to Npc1-KO cells increased the synthesis of D4-deuterated LCFAs (D4-LCFAs) and D4-deuterated ECs (D4-ECs) in a Mal-βCD-dependent manner. Simultaneous addition of D6-deuterated UEC (D6-UEC) and D4-PA promoted the Mal-βCD-dependent synthesis of D6-/D4-ECs, consisting of D6-UEC and D4-PA, D4-deuterated stearic acid, or D4-deuterated myristic acid, in Npc1-KO cells. These results suggest that Mal-βCD helps to maintain normal lipid metabolism by restoring balance among UEC, ECs, and LCFAs through acting on behalf of NPC1 in Npc1-KO cells and may therefore be useful in designing effective therapies for NPC.

Temporal analysis of localization and trafficking of glycolipids

Glycolipid metabolism occurs in the Golgi apparatus, but the detailed mechanisms have not yet been elucidated. We used fluorescently labeled glycolipids to analyze glycolipid composition and localization changes and shed light on glycolipid metabolism. In a previous study, the fatty chain of lactosyl ceramide was fluorescently labeled with BODIPY (LacCer-BODIPY) before being introduced into cultured cells to analyze the cell membrane glycolipid recycling process. However, imaging analysis of glycolipid recycling is difficult because of limited spatial resolution. Therefore, we examined the microscopic conditions that allow the temporal analysis of LacCer-BODIPY trafficking and localization. We observed that the glycolipid fluorescent probe migrated from the cell membrane to intracellular organelles before returning to the cell membrane. We used confocal microscopy to observe co-localization of the glycolipid probe with endosomes and Golgi markers, demonstrating that it recycles mainly through the trans-Golgi network (TGN). Here, a glycolipid recycling pathway was observed that did not require the lipids to pass through the lysosome.

Lipid droplets as ubiquitous fat storage organelles in C. elegans

BACKGROUND

Lipid droplets are a class of eukaryotic cell organelles for storage of neutral fat such as triacylglycerol (TAG) and cholesterol ester (CE). We and others have recently reported that lysosome-related organelles (LROs) are not fat storage structures in the nematode C. elegans. We also reported the formation of enlarged lipid droplets in a class of peroxisomal fatty acid β-oxidation mutants. In the present study, we seek to provide further evidence on the organelle nature and biophysical properties of fat storage structures in wild-type and mutant C. elegans.

RESULTS

In this study, we provide biochemical, histological and ultrastructural evidence of lipid droplets in wild-type and mutant C. elegans that lack lysosome related organelles (LROs). The formation of lipid droplets and the targeting of BODIPY fatty acid analogs to lipid droplets in live animals are not dependent on lysosomal trafficking or peroxisome dysfunction. However, the targeting of Nile Red to lipid droplets in live animals occurs only in mutants with defective peroxisomes. Nile Red labelled-lipid droplets are characterized by a fluorescence emission spectrum distinct from that of Nile Red labelled-LROs. Moreover, we show that the recently developed post-fix Nile Red staining method labels lipid droplets exclusively.

CONCLUSIONS

Our results demonstrate lipid droplets as ubiquitous fat storage organelles and provide a unified explanation for previous studies on fat labelling methods in C. elegans. These results have important applications to the studies of fat storage and lipid droplet regulation in the powerful genetic system, C. elegans.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Membrane traffic and turnover in TRP-ML1-deficient cells: a revised model for mucolipidosis type IV pathogenesis

The lysosomal storage disorder mucolipidosis type IV (MLIV) is caused by mutations in the transient receptor potential-mucolipin-1 (TRP-ML1) ion channel. The "biogenesis" model for MLIV pathogenesis suggests that TRP-ML1 modulates postendocytic delivery to lysosomes by regulating interactions between late endosomes and lysosomes. This model is based on observed lipid trafficking delays in MLIV patient fibroblasts. Because membrane traffic aberrations may be secondary to lipid buildup in chronically TRP-ML1-deficient cells, we depleted TRP-ML1 in HeLa cells using small interfering RNA and examined the effects on cell morphology and postendocytic traffic. TRP-ML1 knockdown induced gradual accumulation of membranous inclusions and, thus, represents a good model in which to examine the direct effects of acute TRP-ML1 deficiency on membrane traffic. Ratiometric imaging revealed decreased lysosomal pH in TRP-ML1-deficient cells, suggesting a disruption in lysosomal function. Nevertheless, we found no effect of TRP-ML1 knockdown on the kinetics of protein or lipid delivery to lysosomes. In contrast, by comparing degradation kinetics of low density lipoprotein constituents, we confirmed a selective defect in cholesterol but not apolipoprotein B hydrolysis in MLIV fibroblasts. We hypothesize that the effects of TRP-ML1 loss on hydrolytic activity have a cumulative effect on lysosome function, resulting in a lag between TRP-ML1 loss and full manifestation of MLIV.

Endogenous versus exogenous fatty acid availability affects lysosomal acidity and MHC class II expression

Although the immune system, inflammation, and cellular metabolism are linked to diseases associated with dyslipidemias, the mechanism(s) remain unclear. To determine whether there is a mechanistic link between lipid availability and inflammation/immune activation, we evaluated macrophage cell lines incubated under conditions of altered exogenous and endogenous lipid availability. Limiting exogenous lipids results in decreased lysosomal acidity and decreased lysosomal enzymatic activity. Both lysosomal parameters are restored with the addition of oleoyl-CoA, suggesting that fatty acids play a role in the regulation of lysosomal function. Cell surface expression of major histocompatibility complex (MHC)-encoded molecules is also decreased in the absence of exogenous lipids. Additionally, we observe decreased gamma-interferon stimulation of cell surface MHC class II. Using cerulenin to limit the endogenous synthesis of fatty acids results in decreased cell surface expression of MHC class II but does not appear to alter lysosomal acidity, suggesting that lysosomal acidity is dependent on exogenous, but not endogenous, fatty acid availability. Testing these conclusions in an in vivo mouse model, we observed statistically significant, diet-dependent differences in lysosomal acidity and MHC class II cell surface expression. Collectively, these data demonstrate a mechanistic link between lipid availability and early events in the immune response.

Flux of Fatty Acids through NPC1 Lysosomes

Niemann-Pick type C (NPC) is an autosomal recessive lipid storage disorder characterized by lysosomal accumulation of cholesterol and gangliosides resulting from a defect in intracellular lipid trafficking. The NPC1 gene encodes a 1278-amino acid integral membrane protein involved in the sub-cellular trafficking of lipids. The exact biological function of NPC1 remains unclear. Recent evidence suggests that NPC1 is a eukaryotic member of the RND permease family of transport proteins, which when expressed in bacteria is capable of transporting fatty acids. The goal of this project was to assess the role of NPC1 in the transport of fatty acids in primary human fibroblasts using normal fibroblasts and fibroblasts from patients with three lysosomal storage diseases: NPC, mucolipidosis IV, and Sandhoff disease. If NPC1 is a fatty acid transporter, we expect to find fatty acid accumulation only in NPC fibroblasts. We used three experimental approaches to assess the role of NPC1 as a fatty acid transporter. First, we evaluated the accumulation versus metabolism of low density lipoprotein-derived oleic acid. Second, we assessed the amount of free fatty acid present after growth in lipoprotein-containing media. Third, we assessed the cellular accumulation of acriflavine, a fluorescent substrate for a number of resistance-nodulation-cell division permease transporters. Our results indicate that fatty acid flux through NPC1-deficient lysosomes is normal.

Hypertriglyceridemia in Lecithin-cholesterol Acyltransferase-deficient Mice Is Associated with Hepatic Overproduction of Triglycerides, Increased Lipogenesis, and Improved Glucose Tolerance

Lecithin-cholesterol acyltransferase deficiency is frequently associated with hypertriglyceridemia (HTG) in animal models and humans. We investigated the mechanism of HTG in the ldlr-/- x lcat-/- (double knockout (dko)) mice using the ldlr-/- x lcat+/+ (knock-out (ko)) littermates as control. Mean fasting triglyceride (TG) levels in the dko mice were elevated 1.75-fold compared with their controls (p < 0.002). Both the very low density lipoprotein and the low density lipoprotein/intermediate density lipoprotein fractions separated by fast protein liquid chromatography were TG-enriched in the dko mice. In vitro lipolysis assay revealed that the dko mouse very low density lipoprotein (d < 1.019 g/ml) fraction separated by ultracentrifugation was a more efficient substrate for lipolysis by exogenous bovine lipoprotein lipase. Post-heparin lipoprotein lipase activity was reduced by 61% in the dko mice. Hepatic TG production rate, determined after intravenous Triton WR1339 injection, was increased 8-fold in the dko mice. Hepatic mRNA levels of sterol regulatory element binding protein-1 (srebp-1) and its target genes acetyl-CoA carboxylase-1 (acc-1), fatty acid synthase (fas), and stearoyl-CoA desaturase-1 (scd-1) were significantly elevated in the dko mice compared with the ko control. The hepatic mRNA levels of LXRalpha (lxralpha) and its target genes including angiopoietin-like protein 3 (angptl-3) in the dko mice were unchanged. Fasting glucose and insulin levels were reduced by 31 and 42%, respectively in the dko mice, in conjunction with a 49% reduction in hepatic pepck-1 mRNA (p = 0.014). Both the HTG and the improved fasting glucose phenotype seen in the dko mice are at least in part attributable to an up-regulation of the hepatic srebp-1c gene.

Suppression of Macrophage Eicosanoid Synthesis by Atherogenic Lipoproteins Is Profoundly Affected by Cholesterol-Fatty Acyl Esterification and the Niemann-Pick C Pathway of Lipid Trafficking

Atheroma macrophages internalize large quantities of lipoprotein-derived lipids. While most emphasis has been placed on cholesterol, lipoprotein-derived fatty acids may also play important roles in lesional macrophage biology. Little is known, however, about the trafficking or metabolism of these fatty acids. In this study, we first show that the cholesterol-fatty acyl esterification reaction, catalyzed by acyl-CoA:cholesterol acyltransferase (ACAT), competes for the incorporation of lipoprotein-derived fatty acids into cellular phospholipids. Furthermore, conditions that inhibit trafficking of cholesterol from late endosomes/lysosomes to the endoplasmic reticulum (ER), such as the amphipathic amine U18666A and the Npc1+/- mutation, also inhibit incorporation of lipoprotein-derived fatty acids into phospholipids. The biological relevance of these findings was investigated by studying the suppression of agonist-induced prostaglandin E(2) (PGE(2)) and leukotriene C(4)/D(4)/E(4) production during lipoprotein uptake by macrophages, which has been postulated to involve enrichment of cellular phospholipids with non-arachidonic fatty acids (NAAFAs). We found that eicosanoid suppression was markedly enhanced when ACAT was inhibited and prevented when late endosomal/lysosomal lipid trafficking was blocked. Moreover, PGE(2) suppression depended entirely on acetyl-LDL-derived NAAFAs, not on acetyl-LDL-cholesterol, and was not due to decreased cPLA(2) activity per se. These data support the following model: lipoprotein-derived NAAFAs traffic via the NPC1 pathway from late endosomes/lysosomes to a critical pool of phospholipids. In competing reactions, these NAAFAs can be either esterified to cholesterol or incorporated into phospholipids, resulting in suppression of eicosanoid biosynthesis. In view of recent evidence suggesting dysfunctional cholesterol esterification in late lesional macrophages, these data predict that such cells would have highly suppressed eicosanoid synthesis, thus affecting eicosanoid-mediated cell signaling in advanced atherosclerosis.

Delayed lysosomal metabolism of lipids in mucolipidosis type IV fibroblasts after LDL-receptor-mediated endocytosis

We specifically probed the low-density lipoprotein-receptor-dependent endosomal/lysosomal pathway of lipid degradation in control and mucolipidosis type IV fibroblasts using either [choline-methyl-14C]sphingomyelin in complex with apolipoprotein E, or cholesteryl [14C]oleate-labelled low-density lipoprotein as a substrate. Mucolipidosis type IV fibroblasts metabolized [14C]sphingomyelin and cholesteryl [14C]oleate significantly more slowly than controls and fibroblasts from patients with Hurler disease or Niemann-Pick disease type C. So far, no lysosomal enzyme deficiency has been reported for mucolipidosis type IV. Rather, the defect in mucolipidosis type IV cells has recently been suggested to be related to intracellular trafficking. Our results suggest that the defect in mucolipidosis type IV also affects the low-density lipoprotein-receptor-mediated endocytosis pathway.

Multidrug permeases and subcellular cholesterol transport

Studies of Niemann-Pick C (NPC) and Tangier diseases have led to the identification of the causative genes, NPC1 and ABCA1, respectively. Characterization of their protein products shows that NPC1 and ABCA1 are permeases that belong to two different superfamilies of efflux pumps, which might be important in subcellular lipid and cholesterol transport.

Difference in substrate specificity between human and mouse lysosomal acid lipase: low affinity for cholesteryl ester in mouse lysosomal acid lipase

Lysosomal acid lipase (LAL) is essential for the intracellular degradation of cholesteryl esters (CE) and triacylglycerols (TG) that are delivered to lysosomes by low density lipoprotein (LDL) receptor mediated endocytosis. We have analysed the difference in the catalytic properties and substrate specificity of human and mouse LALs. LAL activities were measured in human and mouse fibroblasts and in HeLa cells transiently expressing wild-type or site-directed mutant LALs of the two species using the T7 vaccinia system. Cholesteryl esterase and triacylglycerol lipase activities were determined in cellular homogenates with a phospholipid/detergent vesicle assay, an assay frequently used to diagnose human LAL deficiency syndromes, and with LDL particles, a more physiological substrate. Characterisation of human and mouse LAL using these two assays demonstrated marked differences in their TG and CE hydrolysing activities. Compared to human LAL mouse LAL showed a much lower cholesteryl esterase activity in both assays used. The difference was more pronounced in the vesicle assay. The lower cholesteryl esterase activity of mouse LAL did not affect the LDL-CE degradation in intact fibroblasts. The analysis of site-directed mutants suggests a role of the non-conserved cysteine residue at position 240 in cholesteryl esterase activity in human LAL. Our results show a significant difference between human and mouse LAL in their specificity toward cholesteryl esters. The low cholesteryl esterase activity does not result in reduced LDL-cholesterol ester degradation in mouse fibroblasts in situ. In addition, this work emphasises the importance of the physical state of substrates in studies of the specificity and properties of lipolytic enzymes.