Synthesis and hydrolysis of cholesteryl esters by isolated rat-liver lysosomes and cell-free extracts of human lung fibroblasts

Lipids, lysosomes, and autophagy

Lipids are essential components of a cell providing energy substrates for cellular processes, signaling intermediates, and building blocks for biological membranes. Lipids are constantly recycled and redistributed within a cell. Lysosomes play an important role in this recycling process that involves the recruitment of lipids to lysosomes via autophagy or endocytosis for their degradation by lysosomal hydrolases. The catabolites produced are redistributed to various cellular compartments to support basic cellular function. Several studies demonstrated a bidirectional relationship between lipids and lysosomes that regulate autophagy. While lysosomal degradation pathways regulate cellular lipid metabolism, lipids also regulate lysosome function and autophagy. In this review, we focus on this bidirectional relationship in the context of dietary lipids and provide an overview of recent evidence of how lipid-overload lipotoxicity, as observed in obesity and metabolic syndrome, impairs lysosomal function and autophagy that may eventually lead to cellular dysfunction or cell death.

Human platelets have cholesteryl ester hydrolytic activity resulting in esterification of [1-14C]oleate to individual phospholipids of platelets

Human platelet cholesteryl ester hydrolytic (CEH) activity was determined toward cholesteryl [1-14C]oleate resulting in esterification of [1-14C]oleate to individual platelet phospholipids: choline-containing phospholipids (PC); ethanolamine-containing phospholipids (PE); phosphatidylserine (PS); phosphatidylinositol (PI); and sphingomyelin (SPH). Liberation of [1-14C]oleate and esterification of [1-14C]oleate to platelet phospholipids was enhanced by 100 nM iloprost (a stable analogue of prostacyclin that increases platelet cyclic adenosine monophosphate (c-AMP)), inhibited by 30 microM H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide)) (a specific c-AMP dependent protein kinase (CADPK) inhibitor) and 500 microM 2',5' dideoxyadenosine (DDA) (an inhibitor of iloprost-induced rise in platelet c-AMP), but unaffected by 150 mM choloroquine diphosphate. These observations suggest that the CEH activity is mediated by a CADPK phosphorylation of an enzyme with the phosphorylated state representing the active form of the enzyme and that the CEH activity is extralysosomal.

Human platelets have cholesteryl ester hydrolytic activity toward plasma high density lipoproteins

Human platelet cholesteryl ester hydrolytic (CEH) activity was determined toward high density lipoprotein (HDL) labelled with cholesteryl [1-(14)C] oleate resulting in esterilkation of [l-(14)C] oleate to platelet phospholipid. The observed CEH activity was enhanced by 100 nM prostacyclin (PGI(2)), inhibited by 500 μM 2', 3' dideoxyadenosine (DDA), but unaffected by 100 mM chloroquine diphosphate. The CEH activity may represent a mechanism for delivery of other unsaturated fatty acids from HDL to platelets with subsequent modification of the fatty acid composition of platelet phospholipids and potential modification of platelet reactivity.

Intramembrane particles and filipin labelling on the membranes of autophagic vacuoles and lysosomes in mouse liver

Morphologically detectable protein (intramembrane particles) and cholesterol (filipin labelling) in the membranes of autophagic vacuoles and lysosomes were studied in mouse hepatocytes using thin-section and freeze-fracture electron microscopy. Both isolated autophagic vacuoles and lysosomes, and intact tissue blocks were used due to the facts (i) that lysosomes are difficult to recognize in freeze-fracture replicas of intact hepatocytes, and (i) that filipin penetration into the tissue blocks is unsatisfactory. Intramembrane particle density was low in the membranes of early autophagic vacuoles (defined as round-shaped vacuoles in which an inner membrane parallel with the outer limiting membrane was clearly visible). The lysosomal membranes contained considerably more intramembrane particles. Particle-rich lysosomes or other vesicles were observed to fuse with the early autophagic vacuoles. The membranes of nascent autophagic vacuoles with morphologically intact contents were usually not labelled by filipin, whereas the membranes of all other autophagic vacuoles and lysosomes were heavily labelled. The increased cholesterol in the membranes of slightly older autophagic vacuoles is presumably derived from cholesterol-rich lysosomes or other vesicles fusing with the vacuoles and from the degrading organelles inside the autophagic vacuoles.

Fatty acid specificity of the lysosomal acid cholesterol esterase in intact human arterial smooth muscle cells

The fatty-acid specificity of the lysosomal cholesterol esterase was examined in cultured human arterial smooth muscle cells. The lysosomal compartment of cultured cells was enriched with cholesteryl esters by incubation of cells with 0.2 mg/ml low-density lipoprotein and 50 microM chloroquine for 24 h. The hydrolysis of cholesteryl esters was subsequently induced by incubating cells in a medium containing 5% lipoprotein-deficient serum without chloroquine. Cellular cholesteryl ester mass was markedly reduced after 23 h in the lipoprotein-deficient serum. Fatty-acid analysis of cholesteryl esters in cells before and after the 23 h incubation with lipoprotein-deficient serum revealed that polyunsaturated cholesteryl esters (linoleate and arachidonate) were preferentially hydrolyzed compared to cholesteryl oleate or saturated cholesteryl esters. An increase in the ratio of cholesteryl oleate to cholesteryl linoleate was observed even when the cellular activity of acyl-CoA:cholesterol acyltransferase was inhibited with Sandoz Compound 58-035. We conclude that, in human arterial smooth muscle cells, the lysosomal acid cholesterol esterase preferentially hydrolyzes polyunsaturated cholesteryl esters.

Effects of substrate composition on the esterification and hydrolysis activity of lysosomal acid sterol ester hydrolase

Lipid microemulsions with various core and surface lipid compositions were prepared by co-sonication of cholesteryl esters, triolein (TO), egg phosphatidylcholine (egg PC), and cholesterol. The heterogeneous emulsion particle mixture was purified by gel filtration and particles with the size and general organization of low density lipoproteins were obtained. These lipid microemulsion particles were used for studies of the cellular metabolism of lipoprotein-derived cholesterol and cholesteryl esters as catalyzed by the enzyme acid sterol ester hydrolase (EC 3.1.1.13). The hydrolysis of cholesteryl oleate (CO) was more than twice and that of cholesteryl linoleate (CL) more than three times faster than the hydrolysis of cholesteryl stearate (CS) over the temperature range 25-39.6 degrees C. Both the synthesis and hydrolysis of cholesteryl esters were insensitive to the physical state of the microemulsion cores. The synthesis of cholesteryl esters by this enzyme was also insensitive to the ratios of cholesterol and egg PC in the microemulsion surface layers. Incorporation of triolein into the microemulsion cholesteryl ester core slightly increased the rate of cholesteryl ester synthesis. A decreasing fatty acyl chain length (C18:0 to C14:0) and an increasing degree of unsaturation (C18:0 to C18:2) enhanced the synthesis rate. It is suggested that the hydrolysis and synthesis of cholesteryl esters in microemulsions (and lipoproteins) take place only in the particle surface layer and that the rate of catalysis is directly dependent on the amount of substrate in this surface layer.

[3H]cholesterol transfer from microemulsion particles of different sizes to human fibroblasts

A new technique for preparing microemulsion particles of well-defined sizes and compositions is presented. Utilization of these microemulsions is advocated as lipoprotein models in studies of lipid transport and metabolism, rather than the currently used phospholipid-cholesterol vesicles. The emulsion particles consisted of egg phosphatidylcholine and cholesterol as surface lipids and cholesteryl oleate as core lipid. They were prepared by a combined injection and sonication technique and size-separated by a two-step procedure of gel filtration chromatography and density gradient centrifugation. By varying the ratios of core and surface material, particles covering a size range of 20-200 nm in diameter could be produced. The adequacy of these microemulsions as lipoprotein models was tested by studying the transfer of [3H]cholesterol and [14C]cholesterol oleate from the particles to cultured human fibroblasts. Up to a particle size of 100 nm, there was a slight increase of [3H]cholesterol transfer. The transfer of [14C]cholesteryl oleate was very slow, yet measurable. Studies of the exchangeability of cholesterol between the microemulsion core and surface phases indicated that all cholesterol can be transferred from microemulsions to cultured cells as a single pool.

Intracellular processing of exogenously derived non-lipoprotein [3H]cholesterol in normal and mutant human skin fibroblasts deficient in acid sterol ester hydrolase

By studying the incorporation and esterification of non-lipoprotein, free [3H]cholesterol in normal and acid sterol ester hydrolase-deficient human fibroblasts, it was examined whether the esterification reaction of the lysosomal acid sterol ester hydrolase contributed to the formation of cellular [3H]cholesteryl esters. Both the normal and the acid sterol ester hydrolase-deficient cells incorporated exogenous, vesicle-derived free [3H]cholesterol linearly as a function of time. Also, the rate of [3H]cholesteryl ester formation was almost the same in normal and mutant fibroblasts, indicating that the apparent esterification activity of the acid sterol ester hydrolase in normal fibroblasts did not contribute to the formation of [3H]cholesteryl esters in intact cells. To examine whether the incorporated [3H]cholesterol was transported into the endoplasmic reticulum and esterified by the acyl-CoA: cholesterol acyltransferase, the rate of [3H]cholesteryl ester formation was measured in the presence or absence of the acyl-CoA: cholesterol acyltransferase-inhibitor 58-035 (Sandoz Inc.). Results showed that the formation of [3H]cholesteryl esters was reduced markedly when cells were co-incubated with the acyltransferase inhibitor. Maximal inhibition (i.e., 75%) was obtained at an inhibitor concentration of 1 microgram/ml. Since the inhibitor 58-035 is very specific for acyl-CoA: cholesterol acyltransferase, this finding clearly shows that exogenous, exchangeable [3H]cholesterol can reach and mix with the intracellular substrate pool of the enzyme.