The pathway of triacylglycerol synthesis through phosphatidylcholine in Arabidopsis produces a bottleneck for the accumulation of unusual fatty acids in transgenic seeds

Engineering of oilseed plants to accumulate unusual fatty acids (FAs) in seed triacylglycerol (TAG) requires not only the biosynthetic enzymes for unusual FAs but also efficient utilization of the unusual FAs by the host-plant TAG biosynthetic pathways. Competing pathways of diacylglycerol (DAG) and subsequent TAG synthesis ultimately affect TAG FA composition. The membrane lipid phosphatidylcholine (PC) is the substrate for many FA-modifying enzymes (desaturases, hydroxylases, etc.) and DAG can be derived from PC for TAG synthesis. The relative proportion of PC-derived DAG versus de novo synthesized DAG utilized for TAG synthesis, and the ability of each pathway to utilize unusual FA substrates, are unknown for most oilseed plants, including Arabidopsis thaliana. Through metabolic labeling experiments we demonstrate that the relative flux of de novo DAG into the PC-derived DAG pathway versus direct conversion to TAG is ～14/1 in wild-type Arabidopsis. Expression of the Ricinus communis FA hydroxylase reduced the flux of de novo DAG into PC by ～70%. Synthesis of TAG directly from de novo DAG did not increase, resulting in lower total synthesis of labeled lipids. Hydroxy-FA containing de novo DAG was rapidly synthesized, but it was not efficiently accumulated or converted to PC and TAG, and appeared to be in a futile cycle of synthesis and degradation. However, FA hydroxylation on PC and conversion to DAG allowed some hydroxy-FA to accumulate in sn-2 TAG. Therefore, the flux of DAG through PC represents a major bottleneck for the accumulation of unusual FAs in TAG of transgenic Arabidopsis seeds.     

Short‐Term Effects of Dietary Fatty Acids on Muscle Lipid Composition and Serum Acylcarnitine Profile in Human Subjects

In cultured cells, palmitic acid (PA) and oleic acid (OA) confer distinct metabolic effects, yet, unclear, is whether changes in dietary fat intake impact cellular fatty acid (FA) composition. We hypothesized that short‐term increases in dietary PA or OA would result in corresponding changes in the FA composition of skeletal muscle diacylglycerol (DAG) and triacylglycerol (TAG) and/or the specific FA selected for β‐oxidation. Healthy males (N = 12) and females (N = 12) ingested a low‐PA diet for 7 days. After fasting measurements of the serum acylcarnitine (AC) profile, subjects were randomized to either high‐PA (HI PA) or low‐PA/high‐OA (HI OA) diets. After 7 days, the fasting AC measurement was repeated and a muscle/fat biopsy obtained. FA composition of intramyocellular DAG and TAG and serum AC was measured. HI PA increased, whereas HI OA decreased, serum concentration of 16:0 AC (P < 0.001). HI OA increased 18:1 AC (P = 0.005). HI PA was associated with a higher PA/OA ratio in muscle DAG and TAG (DAG: 1.03 ± 0.24 vs. 0.46 ± 0.08, P = 0.04; TAG: 0.63 ± 0.07 vs. 0.41 ± 0.03, P = 0.01). The PA concentration in the adipose tissue DAG (µg/mg adipose tissue) was 0.17 ± 0.02 in those receiving the HI PA diet (n = 6), compared to 0.11 ± 0.02 in the HI oa group (n = 4) (P = 0.067). The relative PA concentration in muscle DAG and TAG and the serum palmitoylcarnitine concentration was higher in those fed the high‐PA diet.     

Challenging of AS160/TBC1D4 Alters Intracellular Lipid milieu in L6 Myotubes Incubated With Palmitate

The Akt substrate of 160 kDa (AS160) is a key regulator of GLUT4 translocation from intracellular depots to the plasma membrane in myocytes. Likely, AS160 also controls LCFAs transport, which requires relocation of fatty acid transporters. The aim of the present study was to determine the impact of AS160 knockdown on lipid milieu in L6 myotubes incubated with palmitate (PA). Therefore, we compared two different settings, namely: 1) AS160 knockdown prior to palmitate incubation (pre‐PA‐silencing, AS160^?/PA); 2) palmitate incubation with subsequent AS160 knockdown (post‐PA‐silencing, PA/AS160^?). The efficiency of AS160 silencing was checked at mRNA and protein levels. The expression and localization of FA transporters were determined using Western Blot and immunofluorescence analyses. Intracellular lipid content (FFA, DAG, TAG, and PL) and FA composition were estimated by GLC, whereas basal palmitate uptake was analyzed by means of scintigraphy. Both groups with silenced AS160 were characterized by a greater expression of FA transporters (FAT/CD36, FATP‐1, 4) which had contributed to an increased FA cellular influx. Accordingly, we observed that post‐PA‐silencing of AS160 resulted in a marked decrement in DAG, TAG, and PL contents, but increased FFA content (PA/AS160^? vs. PA). The opposite effect was observed in the group with pre‐PA‐silencing of AS160 in which AS160 knockdown did not affect the lipid pools (AS160^?/PA vs. PA). Our results indicate that post‐PA‐silencing of AS160 has a capacity to decrease the lipotoxic effect(s) of PA by decreasing the content of lipids (DAG and PL) that promote insulin resistance in myotubes. J. Cell. Physiol. 232: 2373–2386, 2017. © 2016 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals Inc.

     

Neutral Lipid Metabolism Influences Phospholipid Synthesis and Deacylation in Saccharomyces cerevisiae

Establishment and maintenance of equilibrium in the fatty acid (FA) composition of phospholipids (PL) requires both regulation of the substrate available for PL synthesis (the acyl-CoA pool) and extensive PL turnover and acyl editing. In the present study, we utilize acyl-CoA synthetase (ACS) deficient cells, unable to recycle FA derived from lipid deacylation, to evaluate the role of several enzymatic activities in FA trafficking and PL homeostasis in Saccharomyces cerevisiae. The data presented show that phospholipases B are not contributing to constitutive PL deacylation and are therefore unlikely to be involved in PL remodeling. In contrast, the enzymes of neutral lipid (NL) synthesis and mobilization are central mediators of FA trafficking. The phospholipid:DAG acyltransferase (PDAT) Lro1p has a substantial effect on FA release and on PL equilibrium, emerging as an important mediator in PL remodeling. The acyl-CoA dependent biosynthetic activities of NL metabolism are also involved in PL homeostasis through active modulation of the substrate available for PL synthesis. In addition TAG mobilization makes an important contribution, especially in cells from stationary phase, to FA availability. Beyond its well-established role in the formation of a storage pool, NL metabolism could play a crucial role as a mechanism to uncouple the pools of PL and acyl-CoAs from each other and thereby to allow independent regulation of each one.     

Insig Proteins Mediate Feedback Inhibition of Cholesterol Synthesis in the Intestine

Enterocytes are the only cell type that must balance the de novo synthesis and absorption of cholesterol, although the coordinate regulation of these processes is not well understood. Our previous studies demonstrated that enterocytes respond to the pharmacological blockade of cholesterol absorption by ramping up de novo sterol synthesis through activation of sterol regulatory element-binding protein-2 (SREBP-2). Here, we genetically disrupt both Insig1 and Insig2 in the intestine, two closely related proteins that are required for the feedback inhibition of SREBP and HMG-CoA reductase (HMGR). This double knock-out was achieved by generating mice with an intestine-specific deletion of Insig1 using Villin-Cre in combination with a germ line deletion of Insig2. Deficiency of both Insigs in enterocytes resulted in constitutive activation of SREBP and HMGR, leading to an 11-fold increase in sterol synthesis in the small intestine and producing lipidosis of the intestinal crypts. The intestine-derived cholesterol accumulated in plasma and liver, leading to secondary feedback inhibition of hepatic SREBP2 activity. Pharmacological blockade of cholesterol absorption was unable to further induce the already elevated activities of SREBP-2 or HMGR in Insig-deficient enterocytes. These studies confirm the essential role of Insig proteins in the sterol homeostasis of enterocytes.     

Acth increases de novo synthesis of diacylglycerol and translocates protein kinase C in primary cultures of calf adrenal glomerulosa cells

Effects of ACTH on the production of diacylglycerol (DAG) and translocation of protein kinase C were studied in primary cultures of calf adrenal glomerulosa cells. To study DAG production two different labeling protocols were used: (a) cells were prelabeled for 3 days with [2-³H]glycerol before ACTH addition; (b) ACTH and [2-³ H]glycerol were added simultaneously to cells. In both cases, ACTH provoked rapid increases in the labeling of DAG which were maximal in 2 min, dose-dependent, and paralleled by increases in DAG mass. ACTH also increased the labeling of total glycerolipids including phosphatidic acid (PA), phosphatidylinositol, phosphatidylethanolamine, phosphatidylcholine and triacylglycerol. In both labeling protocols, the rates of increase in the labeling of DAG and PA were greater than those of other glycerolipids. Our results indicate that ACTH rapidly increases DAG, at least partly by stimulating the de novo synthesis of PA. In addition, we found that ACTH, like phorbol esters, stimulated the apparent translocation of immunoreactive protein kinase C from the cytosol to the membrane fraction.     

Recruiting a new substrate for triacylglycerol synthesis in plants: the monoacylglycerol acyltransferase pathway

Background

Monoacylglycerol acyltransferases (MGATs) are predominantly associated with lipid absorption and resynthesis in the animal intestine where they catalyse the first step in the monoacylglycerol (MAG) pathway by acylating MAG to form diacylglycerol (DAG). Typical plant triacylglycerol (TAG) biosynthesis routes such as the Kennedy pathway do not include an MGAT step. Rather, DAG and TAG are synthesised de novo from glycerol-3-phosphate (G-3-P) by a series of three subsequent acylation reactions although a complex interplay with membrane lipids exists.

Methodology/Principal Findings

We demonstrate that heterologous expression of a mouse MGAT acyltransferase in Nicotiana benthamiana significantly increases TAG accumulation in vegetative tissues despite the low levels of endogenous MAG substrate available. In addition, DAG produced by this acyltransferase can serve as a substrate for both native and coexpressed diacylglycerol acyltransferases (DGAT). Finally, we show that the Arabidopsis thaliana GPAT4 acyltransferase can produce MAG in Saccharomyces cerevisiae using oleoyl-CoA as the acyl-donor.

Conclusions/Significance

This study demonstrates the concept of a new method of increasing oil content in vegetative tissues by using MAG as a substrate for TAG biosynthesis. Based on in vitro yeast assays and expression results in N. benthamiana, we propose that co-expression of a MAG synthesising enzyme such as A. thaliana GPAT4 and a MGAT or bifunctional M/DGAT can result in DAG and TAG synthesis from G-3-P via a route that is independent and complementary to the endogenous Kennedy pathway and other TAG synthesis routes.     

Diacylglycerol lipase regulates lifespan and oxidative stress response by inversely modulating TOR signaling in Drosophila and C. elegans

Target of rapamycin (TOR) signaling is a nutrient‐sensing pathway controlling metabolism and lifespan. Although TOR signaling can be activated by a metabolite of diacylglycerol (DAG), phosphatidic acid (PA), the precise genetic mechanism through which DAG metabolism influences lifespan remains unknown. DAG is metabolized to either PA via the action of DAG kinase or 2‐arachidonoyl‐sn‐glycerol by diacylglycerol lipase (DAGL). Here, we report that in Drosophila and Caenorhabditis elegans, overexpression of diacylglycerol lipase (DAGL/inaE/dagl‐1) or knockdown of diacylglycerol kinase (DGK/rdgA/dgk‐5) extends lifespan and enhances response to oxidative stress. Phosphorylated S6 kinase (p‐S6K) levels are reduced following these manipulations, implying the involvement of TOR signaling. Conversely, DAGL/inaE/dagl‐1 mutants exhibit shortened lifespan, reduced tolerance to oxidative stress, and elevated levels of p‐S6K. Additional results from genetic interaction studies are consistent with the hypothesis that DAG metabolism interacts with TOR and S6K signaling to affect longevity and oxidative stress resistance. These findings highlight conserved metabolic and genetic pathways that regulate aging.     

Lipogenesis and lipolysis: The pathways exploited by the cancer cells to acquire fatty acids

One of the most important metabolic hallmarks of cancer cells is enhanced lipogenesis. Depending on the tumor type, tumor cells synthesize up to 95% of saturated and mono-unsaturated fatty acids (FA) de novo in spite of sufficient dietary lipid supply. This lipogenic conversion starts early when cells become cancerous and further expands as the tumor cells become more malignant. It is suggested that activation of FA synthesis is required for carcinogenesis and for tumor cell survival. These observations suggest that the enzymes involved in FA synthesis would be rational therapeutic targets for cancer treatment. However, several recent reports have shown that the anti-tumor effects, following inhibition of endogenous FA synthesis in cancer cell lines may be obviated by adding exogenous FAs. Additionally, high intake of dietary fat is reported to be a potential risk factor for development and poor prognosis for certain cancers. Recently it was reported that breast and liposarcoma tumors are equipped for both de novo fatty acid synthesis pathway as well as LPL-mediated extracellular lipolysis. These observations indicate that lipolytically acquired FAs may provide an additional source of FAs for cancer. This review focuses on our current understanding of lipogenic and lipolytic pathways in cancer cell progression.     

Metabolic control of cytosolic‐facing pools of diacylglycerol in budding yeast

Diacylglycerol (DAG) is a key signaling lipid and intermediate in lipid metabolism. Our knowledge of DAG distribution and dynamics in cell membranes is limited. Using live‐cell fluorescence microscopy we investigated the localization of yeast cytosolic‐facing pools of DAG in response to conditions where lipid homeostasis and DAG levels were known to be altered. Two main pools were monitored over time using DAG sensors. One pool was associated with vacuolar membranes and the other localized to sites of polarized growth. Dynamic changes in DAG distribution were observed during resumption of growth from stationary phase, when DAG is used to support phospholipid synthesis for membrane proliferation. Vacuolar membranes experienced constant morphological changes displaying DAG enriched microdomains coexisting with liquid‐disordered areas demarcated by Vph1. Formation of these domains was dependent on triacylglycerol (TAG) lipolysis. DAG domains and puncta were closely connected to lipid droplets. Lack of conversion of DAG to phosphatidate in growth conditions dependent on TAG mobilization, led to the accumulation of DAG in a vacuolar‐associated compartment, impacting the polarized distribution of DAG at budding sites. DAG polarization was also regulated by phosphatidylserine synthesis/traffic and sphingolipid synthesis in the Golgi.      

Using lipidomics to reveal details of lipid accumulation in developing Siberian apricot (Prunus sibirica L.) seed kernels

Siberian apricot seed kernel (SASK) contains a high of 50% oil with suitable fuel properties conformed to biodiesel standard. To date, Prunus sibirica is a novel non‐crop feedstock for biodiesel production in China. Here, oil contents and fatty acid (FA) compositions were identified in developing SASK from AS‐80 and AS‐84, at intervals of 1 week from 3 weeks after anthesis (WAA) to 9 weeks. The major differences in oil content between C18:1 and C18:2 levels were greater among the AS‐80 (32.69/15.48 g/100 g) than among the AS‐84 (25.78/13.15 g/100 g). Subsequently, the SASKs from 4, 6, and 8 WAA, respectively, representing early, middle, and late phases of oil accumulation, were selected as optimal samples for lipidomics analysis. It was notable that 18:1/18:1/18:2, 18:1/18:1/18:3, and 18:2/18:2/18:2 were the prominent compositions in triacylglycerol (TAG), and their higher content found among the AS‐80 was consistent with FA results. Although phosphatidic acid (PA) is directly connected with diacylglycerol (DAG) in Kennedy pathway, we found significant difference between PA and DAG compositions. The resulting molecular species differ in acyl composition depending on whether they were generated via phosphatidylcholine (PC) or Kennedy pathway. By qRT‐PCR analysis, the expression levels of FAD3, PDCT, and DAG‐CPT related to the biosynthesis of polyunsaturated fatty acids (PUFAs) showed a gradual decrease with SASK mature, explaining the drastic change of DAG‐18:3/18:3 content. Additionally, the lipidomics data coupled with qRT‐PCR analysis suggested that phospholipid:DAG acyltransferase may play a critical role in incorporation of PUFAs into sn‐3 of TAG. Our data contribute significantly to understand the underlying mechanisms of lipid accumulation in P. sibirica, and may also present strategies for engineering oil accumulation in oilseed plants.     

Phosphatidic acid accumulation is an early response in the Cf-4/Avr4 interaction

The Cladosporium fulvum (Cf)-4 gene of tomato confers resistance to the fungus C. fulvum, expressing the corresponding avirulence (Avr)4 gene, which codes for an elicitor protein. Little is known about how such mechanisms work, but previous studies have shown that elicitor recognition activates Ca(2+) signalling and protein kinases, such as mitogen-activated protein kinase (MAPK) and calcium-dependent protein kinase (CDPK). Here, we provide evidence that a new signalling component, the lipid second messenger phosphatidic acid (PA), is produced within a few minutes of AVR4/Cf-4 interaction. Using transgenic tobacco cells expressing the tomato Cf-4-resistance gene as a model system, phospholipid signalling pathways were studied by pre-labelling the cells with (32)P(i) and assaying for the formation of lipid signals after challenge with the fungal elicitor AVR4. A dramatic rapid response was an increase in (32)P-PA, together with its metabolic product diacylglycerol pyrophosphate (DGPP). AVR4 increased the levels of PA and DGPP in a Cf-4(+)-, time- and dose-dependent manner, while the non-matching elicitor AVR9 did not trigger any response. In general, PA signalling can be triggered by two different pathways: via phospholipase D (PLD), which generates PA directly by hydrolysing structural phospholipids like phosphatidylcholine (PC), or via PLC, which generates diacylglycerol (DAG) that is subsequently phosphorylated to PA by DAG kinase (DGK). To determine the origin of the AVR4-induced PA formation, a PLD-specific transphosphatidylation assay and a differential (32)P-labelling protocol were used. The results clearly demonstrated that most PA was produced via the phosphorylation of DAG. Neomycin and U73122, inhibitors of PLC activity, inhibited AVR4-induced PA accumulation, suggesting that the increase in DGK activity was because of increased PLC activity producing DAG. Lastly, evidence is provided that PLC signalling and, in particular, PA production could play a role in triggering responses, such as the AVR4-induced oxidative burst. For example, PLC inhibitors inhibited the oxidative burst, and when PA was added to cells, an oxidative burst was induced.     

Increase of hepatic fat accumulation by liver specific expression of Hepatitis B virus X protein in zebrafish

The pathogenesis of fatty liver disease remains largely unknown. Here, we assessed the importance of hepatic fat accumulation on the progression of hepatitis in zebrafish by liver specific expression of Hepatitis B virus X protein (HBx). Transgenic zebrafish lines, GBXs, which selectively express the GBx transgene (GFP-fused HBx gene) in liver, were established. GBX Liver phenotypes were evaluated by histopathology and molecular analysis of fatty acid (FA) metabolism-related genes expression. Most GBXs (66–81%) displayed obvious emaciation starting at 4 months old. Over 99% of the emaciated GBXs developed hepatic steatosis or steatohepatitis, which in turn led to liver hypoplasia. The liver histology of GBXs displayed steatosis, lobular inflammation, and balloon degeneration, similar to non-alcoholic steatohepatitis (NASH). Oil red O stain detected the accumulation of fatty droplets in GBXs. RT-PCR and Q-rt-PCR analysis revealed that GBx induced hepatic steatosis had significant increases in the expression of lipogenic genes, C/EBP-α, SREBP1, ChREBP and PPAR-γ, which then activate key enzymes of the de novo FA synthesis, ACC1, FAS, SCD1, AGAPT, PAP and DGAT2. In addition, the steatohepatitic GBX liver progressed to liver degeneration and exhibited significant differential gene expression in apoptosis and stress. The GBX models exhibited both the genetic and functional factors involved in lipid accumulation and steatosis-associated liver injury. In addition, GBXs with transmissible NASH-like phenotypes provide a promising model for studying liver disease.     

Phosphatidic Acid Sequesters Sec18p from cis‐SNARE Complexes to Inhibit Priming

Yeast vacuole fusion requires the activation of cis‐SNARE complexes through priming carried out by Sec18p/N‐ethylmaleimide sensitive factor and Sec17p/α‐SNAP. The association of Sec18p with vacuolar cis‐SNAREs is regulated in part by phosphatidic acid (PA) phosphatase production of diacylglycerol (DAG). Inhibition of PA phosphatase activity blocks the transfer of membrane‐associated Sec18p to SNAREs. Thus, we hypothesized that Sec18p associates with PA‐rich membrane microdomains before transferring to cis‐SNARE complexes upon PA phosphatase activity. Here, we examined the direct binding of Sec18p to liposomes containing PA or DAG. We found that Sec18p preferentially bound to liposomes containing PA compared with those containing DAG by approximately fivefold. Additionally, using a specific PA‐binding domain blocked Sec18p binding to PA‐liposomes and displaced endogenous Sec18p from isolated vacuoles. Moreover, the direct addition of excess PA blocked the priming activity of isolated vacuoles in a manner similar to chemically inhibiting PA phosphatase activity. These data suggest that the conversion of PA to DAG facilitates the recruitment of Sec18p to cis‐SNAREs. Purified vacuoles from yeast lacking the PA phosphatase Pah1p showed reduced Sec18p association with cis‐SNAREs and complementation with plasmid‐encoded PAH1 or recombinant Pah1p restored the interaction. Taken together, this demonstrates that regulating PA concentrations by Pah1p activity controls SNARE priming by Sec18p.      

Effect of aerobic exercise training on non-alcoholic fatty liver disease induced by a high fat diet in C57BL/6 mice

[Purpose]

The aim of this study was to investigate the effects of aerobic exercise training on a high fat diet (HFD)-induced fatty liver and its metabolic complications in C57BL/6 mice.

[Methods]

Mice at 5-month old (n = 30) were randomly assigned to standard chow (SC + CON, n = 10) and high-fat diet (HFD, n = 20), and they were subjected to SC and HFD, respectively, for 23-week. After 15-week of HFD, mice in the HFD group were further assigned to HFD (HFD + CON, n = 10) or exercise training (HFD + EX, n = 10) groups. The HFD + EX mice were subjected to aerobic treadmill running during the last 8-week of the 23-week HFD course. Outcomes included hepatic steatosis, insulin resistance, and expression of genes involved in mitochondrial function and/or fatty oxidation as well as de novo lipogenesis and/or triacylglycerol (TAG) synthesis.

[Results]

Treadmill running ameliorated impaired glucose tolerance and insulin resistance secondary to the HFD. The beneficial effects of treadmill running were associated with enhanced molecular markers of mitochondrial function and/or fatty acids oxidation (i.e., PPARα and CPT1a mRNAs, pAMPK/AMPK, pACC, and SIRT1 protein) as well as suppressed expression of de novo lipogenesis and/or TAG synthesis (i.e., SREBP1c, lipin1 and FAS mRNAs) in the liver.

[Conclusion]

The current findings suggest that aerobic exercise training is an effective and non-pharmacological means to combat fatty liver and its metabolic complications in HFD-induced obese mice.     

Use of re-esterified palm oils,differing in their acylglycerol structure,in weaning-piglet diets

Re-esterified oils are new fat sources obtained from chemical esterification of acid oils with glycerol (both economically interesting by-products from oil refining and biodiesel industries, respectively). The different fatty acid (FA) positional distribution and acylglycerol composition of re-esterified oils may enhance the apparent absorption of saturated fatty acids (SFA) and, thus, their overall nutritive value. The aim of the present study was to investigate the potential use of re-esterified palm oils, in comparison with their corresponding acid and native oils, and also with an unsaturated fat source in weaning-piglet diets. The parameters assessed were: FA apparent absorption, acylglycerol and free fatty acid (FFA) composition of feces, and growth performance. One-hundred and twenty weaning piglets (average weight of 8.50±1.778 kg) were blocked by initial BW (six blocks) and randomly assigned to five dietary treatments, resulting in four piglets per pen and six replicates per treatment. Dietary treatments were a basal diet supplemented with 10% (as-fed basis) of native soybean oil (SN), native palm oil (PN), acid palm oil (PA), re-esterified palm oil low in mono- (MAG) and diacylglycerols (DAG) (PEL), or re-esterified palm oil high in MAG and DAG (PEH). Results from the digestibility balance showed that SN reached the greatest total FA apparent absorption, and statistically different from PN, PA and PEL (P<0.05). There were no statistical differences among palm-oil sources (P>0.05), but PEH achieved the greatest total FA apparent absorption. Animals fed PEL, despite the fact that PEL oil contained more sn-2 SFA, did not show an improved absorption of SFA (P>0.05). Animals fed PA and PN showed similar apparent absorption coefficients (P>0.05), despite the high FFA content of PA oil. The acylglycerol and FFA composition of feces was mainly composed of FFA. There were no significant differences in growth performance (P>0.05). Results of the present study suggest that, despite the different acylglycerol structure of re-esterified oils, there were no significant differences in digestibility or performance with respect to their corresponding PN and PA oils in weaning-piglet diets.     

Triacylglycerol biosynthesis in yeast

Triacylglycerol (TAG) is the major storage component for fatty acids, and thus for energy, in eukaryotic cells. In this mini-review, we describe recent progress that has been made with the yeast Saccharomyces cerevisiae in understanding formation of TAG and its cell biological role. Formation of TAG involves the synthesis of phosphatidic acid (PA) and diacylglycerol (DAG), two key intermediates of lipid metabolism. De novo formation of PA in yeast as in other types of cells can occur either through the glycerol-3-phosphate- or dihydroxyacetone phosphate-pathways-each named after its respective precursor. PA, formed in two steps of acylation, is converted to DAG by phosphatidate phosphatase. Acylation of DAG to yield TAG is catalyzed mainly by the two yeast proteins Dga1p and Lro1p, which utilize acyl-CoA or phosphatidylcholine, respectively, as acyl donors. In addition, minor alternative routes of DAG acylation appear to exist. Endoplasmic reticulum and lipid particles (LP), the TAG storage compartment in yeast, are the major sites of TAG synthesis. The interplay of these organelles, formation of LP, and enzymatic properties of enzymes catalyzing the synthesis of PA, DAG, and TAG in yeast are discussed in this communication.