Triacylglycerol turnover in Tetrahymena pyriformis. Relation to phospholipid synthesis

Characterization of a second sterol-esterifying enzyme in Toxoplasma highlights the importance of cholesterol storage pathways for the parasite

Lipid bodies are eukaryotic structures for temporary storage of neutral lipids such as acylglycerols and steryl esters. Fatty acyl-CoA and cholesterol are two substrates for cholesteryl ester (CE) synthesis via the ACAT reaction. The intracellular parasite Toxoplasma gondii is incapable of sterol synthesis and unremittingly scavenges cholesterol from mammalian host cells. We previously demonstrated that the parasite expresses a cholesteryl ester-synthesizing enzyme, TgACAT1. In this article, we identified and characterized a second ACAT-like enzyme, TgACAT2, which shares 56% identity with TgACAT1. Both enzymes are endoplasmic reticulum-associated and contribute to CE formation for storage in lipid bodies. While TgACAT1 preferentially utilizes palmitoyl-CoA, TgACAT2 has broader fatty acid specificity and produces more CE. Genetic ablation of each individual ACAT results in parasite growth impairment whereas dual ablation of ACAT1 and ACAT2 is not tolerated by Toxoplasma. ΔACAT1 and ΔACAT2 parasites have reduced CE levels, fewer lipid bodies, and accumulate free cholesterol, which causes injurious membrane effects. Mutant parasites are particularly vulnerable to ACAT inhibitors. This study underlines the important physiological role of ACAT enzymes to store cholesterol in a sterol-auxotrophic organism such as Toxoplasma, and furthermore opens up possibilities of exploiting TgACAT as targets for the development of antitoxoplasmosis drugs.

The dynamic roles of intracellular lipid droplets: from archaea to mammals

During the past decade, there has been a paradigm shift in our understanding of the roles of intracellular lipid droplets (LDs). New genetic, biochemical and imaging technologies have underpinned these advances, which are revealing much new information about these dynamic organelles. This review takes a comparative approach by examining recent work on LDs across the whole range of biological organisms from archaea and bacteria, through yeast and Drosophila to mammals, including humans. LDs probably evolved originally in microorganisms as temporary stores of excess dietary lipid that was surplus to the immediate requirements of membrane formation/turnover. LDs then acquired roles as long-term carbon stores that enabled organisms to survive episodic lack of nutrients. In multicellular organisms, LDs went on to acquire numerous additional roles including cell- and organism-level lipid homeostasis, protein sequestration, membrane trafficking and signalling. Many pathogens of plants and animals subvert their host LD metabolism as part of their infection process. Finally, malfunctions in LDs and associated proteins are implicated in several degenerative diseases of modern humans, among the most serious of which is the increasingly prevalent constellation of pathologies, such as obesity and insulin resistance, which is associated with metabolic syndrome.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Phospholipid biosynthesis in protozoa

Because of the diverse nature of the organisms which are all classed as 'protozoa' (and because of the lack of detailed information on phospholipid metabolism about most of them), it will probably never be possible to generalize phospholipid metabolism to the degree that it has been possible to characterize a mammalian metabolism. Nonetheless, patterns have begun to emerge (i.e. the similarities among the ciliates Entodinium, Paramecium and Tetrahymena) and will not doubt be expanded upon in the future.

Density-dependent variations in the lipid content and metabolism of cultured human keratinocytes

Cultured human foreskin keratinocytes have been utilized extensively to study modulations in protein content during epidermal differentiation. In this study we examined their usefulness as a model system for differentiation-linked changes in lipid content and metabolism. First-to-third passage keratinocytes were grown in 10% fetal calf serum on a mitomycin-treated 3T3 feeder layer and harvested at intervals before, during, and after reaching confluence for determination of lipid, protein, and DNA content. Lipid synthesis, determined as acetate incorporation into lipid, was most active in pre-confluent cultures and at all times closely paralleled the growth activity of the cultures. Post-confluent cultures were characterized by an increase in total lipid content and by increased triglyceride content and synthesis. Pulse-chase studies demonstrated that labeling of the triglyceride pool was labile and suggested that even in post-confluent cultures, triglycerides provide a fatty acid reservoir for phospholipid biosynthesis. A novel band, which co-migrated with monoalkyldiacylglycerol in two solvents systems was present in confluent and post-confluent cultures, but absent in pre-confluent cultures. Sphingolipids constituted less than 10% of total lipid at all stages of growth, and cholesterol sulfate was present only in small quantities. These studies illustrate the relationship of lipid synthesis to growth and demonstrate that human foreskin keratinocytes, cultured under standard conditions, reproduce incompletely the lipid composition of epidermis in vivo.

Alterations of the composition and size of the free fatty acid pool of Tetrahymena responding to low-temperature stress

Cells of Tetrahymena mimbres (formerly T. pyriformis NT-1) in midlogarithmic growth under isothermal conditions (at 39 degrees C) contained a very small, compositionally discrete pool of free fatty acids, principally (60.6% of the total free fatty acid mass) palmitic and stearic acids. The composition, degree of unsaturation, and size of this free fatty acid pool were rapidly (15 min or less) altered in response to chilling. During the acclimation period following chilling to 15 degrees C, the size of the free fatty acid pool increased from a mean value of 15.5 nmol free fatty acid/mumol lipid phosphorus in 39 degrees C cells to 24.2 nmol free fatty acid/mumol lipid phosphorus. The degree of free fatty acid saturation rapidly increased over the initial hour following the onset of hypothermal conditions, but by 24 h the unsaturated free fatty acid/saturated free fatty acid ratio was 0.35 (equivalent to a 2.7-fold increase in unsaturation relative to 39 degrees C controls (unsaturated/saturated ratio = 0.13) and 4.4-fold greater than cells acclimated for 1 h (unsaturated/saturated ratio = 0.08)). By 24 h the percentage of palmitic and stearic acids had decreased to 45.6%. Similar, and in some instances more pronounced, changes were observed to occur in triacylglycerol-bound fatty acids. Modulation of steady-state free fatty acid composition could also be achieved by the addition of exogenous fatty acids to the growth medium. The ability to manipulate the level of intracellular free fatty acids should prove to be a valuable experimental tool in determining how specific fatty acids regulate various lipid-modifying enzymes.

Trypanosoma cruzi: changes in lipid composition during aging in culture

Changes in lipid composition and fatty acid distribution in lipid fractions from total extracts of Trypanosoma cruzi were studied in culture from Day 2 to Day 14. This comprises the phases of exponential, stationary, and declining growth. Total phospholipid content decreased steadily during the three culture phases due to the marked reduction of phosphatidylcholine. Phosphatidylethanolamine increased during the exponential and declining phases. Thus, the final phosphatidylethanolamine/phosphatidylcholine ratio was higher than that determined on the second day. Sterols and acylglycerides increased as cultures aged. Fatty acid composition of different fractions varied during aging: phosphatidylcholine and phosphatidylethanolamine presented an increase of saturated and reduction of polyunsaturated (linoleic) acids, while for lysophosphatidylcholine and acylglycerides, the opposite change occurred. The modifications described may produce reduction of membrane fluidity and indicate that lipids participate actively in the adaptation of T. cruzi to the environmental changes produced by aging in culture.

Lipid metabolism in fungi

So far, reviews that have appeared on fungal lipids present data mainly on the lipid composition of these organisms and the influence of lipids on their physiology. These reviews provide little information about the enzymes of lipid metabolism in these organisms and it is assumed, by most workers, that lipid synthesis in all fungi takes place as in Saccharomyces cervesiae, the only fungus in which the complete pathways of phospholipid biosynthesis have been worked out. During the last few years, literature has accumulated on lipid metabolic enzymes of other fungi, as investigators became increasingly interested in this area of research. The present review, after an introduction, will be divided into different sections and each section will deal, comparatively, with various aspects of fungal lipid metabolism and physiology. This review will, therefore, bring out the differences or similarities of lipid metabolism in diverse fungal species.

Incorporation of [1-14C]octadecanol into the lipids of Leishmania donovani

After incubation of stationary phase Leishmania donovani with [1-14C]octadecanol, about 70% of the precursor was taken up within 3 hr. Wax esters and acyl moieties of glycerolipids contained most of the 14C-activity from 3 to 6 hr, because octadecanol was partly oxidized to stearate. Ether moieties were only weakly labeled. After 40 hr, 1-0-alkyl and 1-0-alk-1'-enyl diacylglycerols as well as 1-0-alkyl and 1-0-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamines contained nearly all of the radioactivity. Most of the label in the neutral ether lipids was located in the alkyl ether side chain, whereas, in the phosphatidylethanolamine fraction, most of the label was found in the alkenyl ether side chain. Administration of 1-0-[1-14C]hexadecyl glycerol resulted in rapid labeling of the vinyl ether side chain of phosphatidylethanolamine plasmalogen (1 hr) increasing further at 2.5 hr. Most of the radioactivity in the alkoxy diacylglycerols was found in the 1-0-alkyl moiety.

Levels of polyunsaturated fatty acids in lymphocytes of rats fed on diets varying in polyunsaturated fatty acid content

1. When weanling rats were fed on a diet containing 0.1 g/kg of the diet as polyunsaturated fatty acid, it was found that after 2 weeks the level of linoleate in the lymphocyte lipids was 56 mg/g total fatty acids, as compared with a level of 138 mg/g in rats on a normal diet (P less than 0.005). Similar levels were obtained from rats which had been fed for up to 16 weeks on the deficient diet, but in a group killed after 28 weeks on the diet level was found to be only 20 mg/g total fatty acids. The arachidonate level was found to be approximately 220 mg/g total fatty acids, regardless of whether the rats were fed on a diet deficient in linoleate for up to 16 weeks or on a normal diet. In the group of rats killed after 28 weeks on the linoleate deficient diet, however, the arachidonate level was only 60 mg/g total fatty acids. 2. Percentage values for total fatty acids are given for plasma, adipose tissue, and lymphocytes for rats on normal and experimental diets. 3. Scatter diagrams of the levels of linoleate v. arachidonate in the lymphocyte total fatty acids showed no correlation between the levels of the two acids (r 0.05), but similar plots of linoleate and oleate levels showed an inverse correlation (r -0.68).

Triaglycerol metabolism in Saccharomyces cerevisiae. Relation to phospholipid synthesis

The acylglycerol content of Saccharomyces cerevisiae has been examined during cellular growth. The cells maintained a constant amount of phospholipid and diacylglycerol throughout growth. Triacylglycerol content fell in the early exponential phase of growth and then increased sharply upon entry of the culture into the stationary growth phase. Pulse-chase experiments with [1-14C]oleic acid and [2-3H]- and [1-14C]glycerol indicated that the triacylglycerol molecule was utilized for phospholipid synthesis in early exponential phase probably through a diacylglycerol intermediate. A substantial turnover of phospholipid during growth was also apparent. No role for the triacylglycerol could be found in regulating the fatty acid species of the phospholipid nor in the storage of fatty acid for energy metabolism.

Oleosomes (Spherosomes) from Daucus carota suspension culture cells

Isolated oleosomes from Daucus carota L. cells are lipid droplets consisting mainly of triacylglycerols (>97%) and very little protein (1-2%). The boundary between the lipid phase and the cytosol, which is visible on electron micrographs, is not built up by a true phospholipid-containing unit or half unit membrane. Enzymatic activities of lipid metabolism were not found to be associated with oleosomes with the exception of very low (contaminating) acyl-CoA:1,2-diacylglycerol acyltransferase (EC 2.3.1.20) and relatively high acyl-CoA hydrolase (EC 3.1.2.2) activities. The triacylglycerols exhibited a half life time of about 70 h, which is below the generation time of the cells (80-90 h). The fatty acid pattern of triacylglycerols was very similar to that of polar cellular membrane lipids.

The effect of phosphatidylcholine depletion on biochemical and physical properties of a Neurospora crassa membrane mutant

By using the choline starvation process it is possible to deplete the membranes of Neurospora crassa choline auxotroph chol-1 of phosphatidylcholine, without affecting the viability of germinated spores or whole mycelium. Spin label probes were used to examine the possible dependence of the physical state of cellular lipids on the presence of phosphatidylcholine in the membranes. Increased freedom of rotational motion of lipid soluble probes was regularly detected in choline-starved mycelium. The accumulation of neutral lipids (mostly triglycerides) in bulk form was also observed during the choline starvation process. The experiments with isolated and separated lipid classes indicated that the observed increase in fluidity of lipids in choline-starved mycelium is partly due to the difference in physical properties between bulk lipids and membrane lipids. Spin label probe 2N4 (2-propyl-2,5,5-trimethyloxazolidine-N-oxyl), which can partition at the membrane-water interface, exhibited easier partitioning among membrane lipids of choline-starved mycelium.

Isolation and characterization of lipid globules from the zoospores of Blastocladiella emersonii

Lipid globules were isolated and characterized both chemically and morphologically. They were composed mainly of triglyceride and free sterol, which accounted for over 90% of the total globule content. Smaller amounts of diglyceride, carotenoid, free fatty acid, phospholipid and protein were found. No sterol esters or monoglycerides were detected. Morphologically, the isolated lipid globules resembled the lipid globules in situ. They were spherical, 0.4-1.5 mum in diameter and lacked a trilaminar membrane.

Molecular control of membrane properties during temperature acclimation. Fatty acid desaturase regulation of membrane fluidity in acclimating Tetrahymena cells

This is a study of the molecular mechanisms employed by Tetrahymena pyriformis to change the lipid composition and thereby the fluidity of its various membranes during temperature acclimation. By quantitatively measuring the intramembrane particle aggregation using freeze-fracture electron microscopy, membrane physical properties in 39.5 degrees C grown cells shifted to 15 degrees C were found to be correlated with the degree of phospholipid fatty acid desaturation. Alteration of the phospholipid polar head group distribution from that of 39.5 degrees C-grown cells to the significantly different pattern of 15 degrees C grown cells appeared not to be of critical importance in the acclimation process. Changes in fatty acid desaturation during acclimation from high to low temperatures and vice versa were analyzed using normal cells and cells fed large amounts of polyunsaturated fatty acids. Fatty acid desaturase activity corresponded to the degree of membrane fluidity but not to the cell temperature. All evidence was compatible with the hypothesis that membrane fluidity is self-regulating, with the action of fatty acid desaturases being modulated by the physical state of their membrane environment.

Molecular control of membrane properties during temperature acclimation. Membrane fluidity regulation of fatty acid desaturase action?

Further studies on the molecular mechanisms of temperature acclimation have been carried out using the ciliate Tetrahymena pyriformis. The most prominent change in lipid metabolism during acclimation to high temperature--depression of fatty acid desaturase activity--could be simulated by supplementing the growth medium of isothermally-grown cells with polyunsaturated fatty acids. Such cells resisted the membrane-fluidizing effect of the incorporated exogenous acids by increased use of de novo synthesized saturated acids in their phospholipids. The data support the conclusions arising from earlier experiments with temperature-shifted cells (Martin, C.E., Hiramitsu, K., Kitajima, Y., Nozawa, Y., Skriver, L., and Thompson, G.A., Jr. (1976), Biochemistry 15), showing that, when membrane fluidity increased to a superoptimal level, the activity of membrane-associated fatty acid desaturases was decreased. Since the reaction is controlled by membrane fluidity, rather than temperature per se, we postulate that it is the general mechnaism employed by cells adjusting to any fluidity-modifying factor, such as cations, drugs, etc.