Influence of chain length of pyrene fatty acids on their uptake and metabolism by Epstein-Barr-virus-transformed lymphoid cell lines from a patient with multisystemic lipid storage myopathy and from control subjects

Fat in the skin: Triacylglycerol metabolism in keratinocytes and its role in the development of neutral lipid storage disease

Keratinocyte differentiation is essential for skin development and the formation of the skin permeability barrier. This process involves an orchestrated remodeling of lipids. The cleavage of precursor lipids from lamellar bodies by β-glucocerebrosidase, sphingomyelinase, phospholipases and sterol sulfatase generates ceramides, non-esterified fatty acids and cholesterol for the lipid-containing extracellular matrix, the lamellar membranes in the stratum corneum. The importance of triacylglycerol (TAG) hydrolysis for the formation of a functional permeability barrier was only recently appreciated. Mice with defects in TAG synthesis (acyl-CoA:diacylglycerol acyltransferase-2-knock-out) or TAG catabolism (comparative gene identification-58, -CGI-58-knock-out) develop severe permeability barrier defects and die soon after birth because of desiccation. In humans, mutations in the CGI-58 gene also cause (non-lethal) neutral lipid storage disease with ichthyosis. As a result of defective TAG synthesis or catabolism, humans and mice lack ω-(O)-acylceramides, which are essential lipid precursors for the formation of the corneocyte lipid envelope. This structure plays an important role in linking the lipid-enriched lamellar membranes to highly cross-linked corneocyte proteins. This review focuses on the current knowledge of biochemical mechanisms that are essential for epidermal neutral lipid metabolism and the formation of a functional skin permeability barrier.

Lipolysis - a highly regulated multi-enzyme complex mediates the catabolism of cellular fat stores

Lipolysis is the biochemical pathway responsible for the catabolism of triacylglycerol (TAG) stored in cellular lipid droplets. The hydrolytic cleavage of TAG generates non-esterified fatty acids, which are subsequently used as energy substrates, essential precursors for lipid and membrane synthesis, or mediators in cell signaling processes. Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, it is most abundant, however, in white and brown adipose tissue. Over the last 5years, important enzymes and regulatory protein factors involved in lipolysis have been identified. These include an essential TAG hydrolase named adipose triglyceride lipase (ATGL) [annotated as patatin-like phospholipase domain-containing protein A2], the ATGL activator comparative gene identification-58 [annotated as α/β hydrolase containing protein 5], and the ATGL inhibitor G0/G1 switch gene 2. Together with the established hormone-sensitive lipase [annotated as lipase E] and monoglyceride lipase, these proteins constitute the basic "lipolytic machinery". Additionally, a large number of hormonal signaling pathways and lipid droplet-associated protein factors regulate substrate access and the activity of the "lipolysome". This review summarizes the current knowledge concerning the enzymes and regulatory processes governing lipolysis of fat stores in adipose and non-adipose tissues. Special emphasis will be given to ATGL, its regulation, and physiological function.

Pyrene-labeled lipids as tools in membrane biophysics and cell biology

Pyrene is one of the most frequently used lipid-linked fluorophores. Its most characteristic features are a long excited state lifetime and (local) concentration-dependent formation of excimers. Pyrene is also hydrophobic and thus does not significantly distort the conformation of the labeled lipid molecule. These characteristics make pyrene lipids well-suited for studies on a variety of biophysical phenomena like lateral diffusion, inter- or transbilayer movement of lipids and lateral organization of membranes. Pyrene lipids have also been widely employed to determine protein binding to membranes, lipid conformation and the activity of lipolytic enzymes. In cell biology, pyrene lipids are promising tools for studies on lipid trafficking and metabolism, as well as for microscopic mapping of membrane properties. The main disadvantage of pyrene lipids is the relatively large size of the fluorophore. Another disadvantage is that they require UV-excitation, which is not feasible with all microscopes.

Cellular differentiation and I-FABP protein expression modulate fatty acid uptake and diffusion

The effect of cellular differentiation on fatty acid uptake and intracellular diffusion was examined in transfected pluripotent mouse embryonic stem (ES) cells stably expressing intestinal fatty acid binding protein (I-FABP). Control ES cells, whether differentiated or undifferentiated, did not express I-FABP. The initial rate and maximal uptake of the fluorescent fatty acid, 12-(N-methyl)-N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-octadec anoic acid (NBD-stearic acid), was measured in single cells by kinetic digital fluorescence imaging. I-FABP expression in undifferentiated ES cells increased the initial rate and maximal uptake of NBD-stearic acid 1.7- and 1.6-fold, respectively, as well as increased its effective intracellular diffusion constant (Deff) 1.8-fold as measured by the fluorescence recovery after photobleaching technique. In contrast, ES cell differentiation decreased I-FABP expression up to 3-fold and decreased the NBD-stearic acid initial rate of uptake, maximal uptake, and Deff by 10-, 4.7-, and 2-fold, respectively. There were no significant differences in these parameters between the differentiated control and differentiated I-FABP-expressing ES cell lines. In summary, differentiation and expression of I-FABP oppositely modulated NBD-stearic acid uptake parameters and intracellular diffusion in ES cells.

Degradation of pyrene-labelled phospholipids by lysosomal phospholipases in vitro. Dependence of degradation on the length and position of the labelled and unlabelled acyl chains

The hydrolysis of pyrenylacyl phosphatidylcholines (PyrnPCs)(n indicates the number of aliphatic carbons in the pyrene-chain) by crude lysosomal phospholipases in vitro was investigated. PyrnPCs consist of several sets in which the length of the pyrene-labelled or the unlabelled acyl chain, linked to the sn-1 or sn-2 position, was systematically varied. Lysophosphatidylcholine and fatty acid were the only fluorescent breakdown products detected, thus indicating that PyrnPCs were degraded by A-type phospholipases and lysophospholipases. Of these, mainly A1-type phospholipases appear to be involved, as determined from the relative amounts of labelled fatty acid and lysolipid released from the positional isomers. Based on the effects of the length and position of the pyrene-labelled and unlabelled chains it is suggested that (1) the lysosomal A-type phospholipases acting on PyrnPCs recognize the carboxy-terminal part of the lipid acyl chains and (2) the relevant part of the binding site is relatively narrow. Thus phospholipids with added bulk in the corresponding region, such as those that are peroxidized and polymerized, may not be good substrates for the lysosomal phospholipases mentioned. The impaired hydrolysis of the most hydrophobic PyrnPCs indicates that lysosomal phospholipases may not be able to penetrate significantly into the substrate interphase, but upward movement of the lipid may be required for efficient hydrolysis. Finally, the rate of hydrolysis of many pyrenyl derivatives was found to be comparable to that of a natural phosphatidylcholine species, both in micelles and in lipoprotein particles, indicating that these derivatives can be used as faithful reporters of lysosomal degradation of natural lipids in vivo and in vitro.

The turnover of cytoplasmic triacylglycerols in human fibroblasts involves two separate acyl chain length-dependent degradation pathways

Cultured fibroblasts from patients affected with the genetic metabolic disorder named neutral lipid storage disease (NLSD) exhibit a dramatic accumulation of cytoplasmic triacylglycerols (Radom, J., Salvayre, R., Nègre, A., Maret, A., and Douste-Blazy, L. (1987) Eur. J. Biochem. 164, 703-708). We compared here the metabolism of radiolabeled short-, medium- and long-chain fatty acids in these cells. Short/medium-chain fatty acids (C4-C10) were incorporated into polar lipids (60-80%) and triacylglycerols (20-40%) at a lower rate (5-10 times lower) than long-chain fatty acids. Pulse-chase experiments allowed to evaluate the degradation rate of cytoplasmic triacylglycerols in normal and NLSD fibroblasts and to discriminate between two catabolic pathways of cytoplasmic triacylglycerols. Short/medium-chain (C4-C10) triacylglycerols were degraded at a normal rate in NLSD fibroblasts, whereas long-chain (C12 and longer) triacylglycerols remained undegraded. These data are confirmed by mass analysis. The use of diethylparanitrophenyl phosphate (E600) and parachloromercuribenzoate (PCMB) inhibitors allows to discriminate between the two triacylglycerol degradation pathways. E600 inhibited selectively the in situ degradation of short/medium-chain triacylglycerols without inhibition of the degradation of long-chain triacylglycerols, whereas PCMB inhibited selectively the in situ hydrolysis of long-chain triacylglycerols without affecting the degradation of long-chain triacylglycerols. This was correlated with the in vitro properties of cellular triacylglycerol-hydrolyzing enzymes characterized by their substrate specificity and their susceptibility to inhibitors; the neutral lipase specific to long-chain triacylglycerols is inhibited by PCMB, but not by E600, in contrast to short/medium-chain lipase, which is inhibited by E600 but not by PCMB. The data of in vitro and in situ experiments suggest the existence in fibroblasts of two separate acyl chain length-dependent pathways involved in the degradation of cytoplasmic triacylglycerols, one mediated by a neutral long-chain lipase and another one mediated by a short/medium-chain lipase.

Cellular uptake and catabolism of high-density-lipoprotein triacylglycerols in human cultured fibroblasts: degradation block in neutral lipid storage disease

High-density lipoprotein (HDL)-[3H]triolein (i.e. [3H]triolein incorporated into reconstituted HDL) was taken up by cultured fibroblasts through an apparently saturable process, competitively inhibited by non-labelled HDL and independent of the LDL receptor. Using 125I-HDL and HDL-[3H]triolein, binding experiments (at 0 degrees C) followed by a short-time 'chase' at 37 degrees C showed that 125I radioactivity was rapidly released in the culture medium (as trichloroacetic acid-precipitable material), whereas 3H radioactivity remained associated with the cell. The cell-associated HDL-[3H]triolein was rapidly degraded in normal fibroblasts, and the liberated [3H]oleic acid was incorporated into newly biosynthesized phospholipids. In Wolman-disease fibroblasts HDL-[3H]triolein was degraded at a normal rate, and thus independently of the lysosomal compartment. In contrast, the degradation of HDL-[3H]triolein was blocked in fibroblasts from Neutral Lipid Storage Disease (NLSD), similarly to that of endogenously biosynthesized triacylglycerols [Radom, Salvayre, Nègre, Maret and Douste-Blazy (1987) Eur. J. Biochem. 164, 703-708]. Trypsin-treated HDL-[3H]triolein was also taken up by cells and degraded quite similarly to HDL-[3H]triolein. In conclusion, all these data taken together suggest that HDL-[3H]triolein is: (i) associated with the cell through a process independent of intact apolipoprotein (apo) As, thus probably independent of an apoA-receptor-mediated uptake; (ii) internalized by cells, whereas 125I-apoAs are released in the culture medium; (iii) directed to the same non-lysosomal catabolic pool (blocked in NLSD) as for endogenously biosynthesized triacylglycerols.

Use of pyrenemethyl laurate for fluorescence-based determination of lipase activity in intact living lymphoblastoid cells and for the diagnosis of acid lipase deficiency

Pyrenemethyl laurate (PMLes), a fluorogenic substrate for determining in vitro lipase activity [Nègre, Salvayre, Dagan and Gatt (1989) Biochim. Biophys. Acta 1006, 84-88], has been administered to cultured lymphoblastoid cells from normal subjects and from a patient affected with Wolman disease, which is characterized by a deficiency of lysosomal acid lipase. The intracellular degradation of PMLes was dependent on the mode of administration of the substrate into the cells, and occurred by two separate pathways involving lysosomal and extra-lysosomal hydrolases. PMLes incorporated into LDL was taken up by normal lymphoblastoid cells through the apolipoprotein-B/E-receptor-mediated pathway and degraded in the lysosomal compartment, as suggested by the degradation block in Wolman cells. In contrast, when PMLes dissolved in 2% dimethyl sulphoxide was added directly to the culture medium, its hydrolysis was similar in lymphoblastoid cells from controls and from patients affected with Wolman disease, neutral lipid storage disease or familial hypercholesterolaemia. This suggested that the administered PMLes was degraded by a non-lysosomal enzyme which is not deficient in Wolman cells. This enzyme also differs from the neutral lipase system which is deficient in lymphoblastoid cells from patients with neutral lipid storage disease. When pyrenemethanol was administered directly to the cell culture, it was only poorly acylated and was rapidly released into the culture medium. These results and the fluorescence properties of PMLes ('monomeric' emission in a hydrophobic environment and 'excimeric' emission in a hydrophilic environment) and pyrenemethanol ('monomeric' emission in a hydrophilic environment) allowed us to design a 'direct reading' procedure by monitoring (without any lipid extraction) the fluorescence of intact living cells and that of the culture medium during pulse-chase experiments. This method allowed the direct evaluation of the time course of in situ degradation of PMLes. In pulse-chase experiments with LDL-PMLes, the fluorescence of normal cells decreased relatively rapidly with time whereas the fluorescence of the culture medium increased concomitantly. With Wolman cells, the cellular fluorescence decreased only very slightly, whereas that of the culture medium remained at the basal level; this demonstrates the catabolic block in intact living cells from patients with Wolman disease. In vitro degradation of PMLes indicated the existence of two PMLes-degrading enzymes in lymphoblastoid cell homogenates: one is the acid lipase which is involved in PMLes degradation in the lysosomal compartment (and is deficient in Wolman cells), while the second is a cytoplasmic enzyme (not deficient in Wolman cells).

Cytoplasmic triacylglycerols and cholesteryl esters are degraded in two separate catabolic pools in cultured human fibroblasts

The sources and the catabolic pathways of cytoplasmic pools of triacylglycerols and cholesteryl esters have been comparatively investigated in cultured fibroblasts from normal subjects and from patients affected with neutral lipid storage disease (NLSD) and Wolman disease (WD). (i) Endogenously biosynthesized triacylglycerols and cholesteryl esters were degraded extra-lysosomally since they were catabolized at similar rates in normal and in WD fibroblasts. In NLSD fibroblasts, the degradation of endogenous triacylglycerols was severely deficient, whereas that of endogenous cholesteryl esters was in the normal range. (ii) Reconstituted high density lipoproteins (HDL) containing radiolabelled [3H]triolein and cholesteryl [14C]oleate were taken up by cultured fibroblasts and rapidly degraded in a non-lysosomal compartment. In NLSD fibroblasts the degradation of HDL-[3H]triolein was blocked whereas that of HDL-[14C]cholesteryl oleate was in the normal range. These data suggest that: (i) the cytoplasmic pools of triacylglycerols and cholesteryl esters originate from HDL uptake and from endogenous biosynthesis as well; (ii) cytoplasmic (non-lysosomal) triacylglycerols and cholesteryl esters are degraded by two separate catabolic pathways.

A novel fluorescent fatty acid, 5-methyl-BDY-3-dodecanoic acid, is a potential probe in lipid transport studies by incorporating selectively to lipid classes of BHK cells

The 5-methyl-BDY-3-dodecanoic acid (B12FA) labelling of BHK cell lipids was analyzed by thin layer and reverse phase column chromatography. Incorporation to phospholipids was selective: over 90% of B12FA label was enriched in phosphatidylcholine. The major molecular species of PC was that containing palmitate as the unlabelled fatty acid. Small amounts of label was also found in other phosphoglycerides, but not in sphingomyelin. Triglycerides and diglycerides constituted the main B12FA-labelled neutral lipid classes; however, no label was found in cholesterol esters. B12FA was degraded to shorter homologues, which had significantly slower lipid incorporation rates. B12FA-labelled cells displayed in a microscope initially green reticular type fluorescence, but later red spherical structures, representing neutral lipid droplets, could also be seen. It is concluded that B12FA does not incorporate indiscriminately to all lipid classes of BHK cells, but is enriched to PC, diglycerides and triglycerides, which could be utilized in studies on lipid transport as well as metabolism.

Uptake and degradation of several pyrenesphingomyelins by skin fibroblasts from control subjects and patients with Niemann-Pick disease. Effect of the structure of the fluorescent fatty acyl residue

Three fluorescent analogues of sphingomyelin (SPM), each containing pyrene in the fatty acyl residue, were synthesized and employed for the study of their mode of uptake by, and degradation within, intact cultured human skin fibroblasts. These were prepared by condensing sphingosylphosphocholine and the following fatty acids: pyrenedodecanoic acid (P12), pyrenesulphonylaminoundecanoic acid (PSA11) and pyrenepropenoic acid (P3:1). The cell association and catabolism of these SPM analogues by normal, Niemann-Pick-disease-Type-A and low-density-lipoprotein (LDL)-receptor-negative familial hypercholesterolaemia fibroblasts were investigated and compared with the metabolism of [cholinemethyl-14C]sphingomyelin. The catabolism of the fluorescent derivatives was monitored by measuring the appearance of the corresponding fluorescent ceramides. Two modes of uptake and degradation patterns were observed. Thus P12-SPM and radiolabelled SPM were taken up by LDL-receptor-mediated endocytosis when incubated with serum-containing medium, this conclusion being supported by the very low uptake by familial-hypercholesterolaemia fibroblasts, which lack the apolipoprotein-B/E receptor. After uptake, these compounds were metabolically degraded solely by the lysosomal sphingomyelinase, as evidenced by the fact that more than 98% of the SPM remained undegraded in Niemann-Pick-disease cells. By contrast, PSA11- and P3:1-SPMs were taken up by a receptor-independent endocytic pathway, as indicated by the similar rates of uptake in control and familial-hypercholesterolaemia cells in the absence or presence of fetal-calf serum in the culture medium. The degradation of PSA11-SPM and P3:1-SPM was brought about, in the main, by the lysosomal sphingomyelinase, but also by a yet uncharacterized process. The latter catabolic pathway, active in Niemann-Pick-disease-Type-A fibroblasts, seems to differ from the neutral Mg2(+)-dependent sphingomyelinase whose activity was undetectable in homogenates of skin fibroblasts. The present study emphasizes the influence of the structure of the fatty acyl moiety of SPM on its association with lipoproteins and/or cell membranes and on its intracellular routing and metabolic degradation.