Effect of hypophysectomy and replacement therapy on fatty acid metabolism in the rat testis

1. The influence of pituitary gonadotrophins and of testosterone on the conversion of linoleic acid into other polyunsaturated fatty acids by rat testicular tissue was studied. 2. In immature hypophysectomized rats, follicle-stimulating hormone caused a threefold increase in the incorporation of radioactivity from [1-(14)C]linoleic acid into testicular lipids; the distribution of (14)C in the polyunsaturated fatty acids, however, was not significantly affected. 3. In mature hypophysectomized rats, the hormonal treatments had less pronounced effects on (14)C incorporation into testicular lipids, but caused a significant increase in the percentage of (14)C incorporated into polyunsaturated fatty acids of the omega-6 series, luteinizing hormone and testosterone having the more pronounced influences. 4. A time-course study of the appearance of radioactivity in the ejaculated spermatozoa of rabbits, after they had been given a tracer dose of [1-(14)C]linoleic acid, indicated that incorporation of radioactivity into spermatozoa occurred during all stages of spermatogenesis.     

Development of marker-free transgenic Jatropha plants with increased levels of seed oleic acid

Background

Jatropha curcas is recognized as a new energy crop due to the presence of the high amount of oil in its seeds that can be converted into biodiesel. The quality and performance of the biodiesel depends on the chemical composition of the fatty acids present in the oil. The fatty acids profile of the oil has a direct impact on ignition quality, heat of combustion and oxidative stability. An ideal biodiesel composition should have more monounsaturated fatty acids and less polyunsaturated acids. Jatropha seed oil contains 30% to 50% polyunsaturated fatty acids (mainly linoleic acid) which negatively impacts the oxidative stability and causes high rate of nitrogen oxides emission.

Results

The enzyme 1-acyl-2-oleoyl-sn-glycero-3-phosphocholine delta 12-desaturase (FAD2) is the key enzyme responsible for the production of linoleic acid in plants. We identified three putative delta 12 fatty acid desaturase genes in Jatropha (JcFAD2s) through genome-wide analysis and downregulated the expression of one of these genes, JcFAD2-1, in a seed-specific manner by RNA interference technology. The resulting JcFAD2-1 RNA interference transgenic plants showed a dramatic increase of oleic acid (> 78%) and a corresponding reduction in polyunsaturated fatty acids (< 3%) in its seed oil. The control Jatropha had around 37% oleic acid and 41% polyunsaturated fatty acids. This indicates that FAD2-1 is the major enzyme responsible for converting oleic acid to linoleic acid in Jatropha. Due to the changes in the fatty acids profile, the oil of the JcFAD2-1 RNA interference seed was estimated to yield a cetane number as high as 60.2, which is similar to the required cetane number for conventional premium diesel fuels (60) in Europe. The presence of high seed oleic acid did not have a negative impact on other Jatropha agronomic traits based on our preliminary data of the original plants under greenhouse conditions. Further, we developed a marker-free system to generate the transgenic Jatropha that will help reduce public concerns for environmental issues surrounding genetically modified plants.

Conclusion

In this study we produced seed-specific JcFAD2-1 RNA interference transgenic Jatropha without a selectable marker. We successfully increased the proportion of oleic acid versus linoleic in Jatropha through genetic engineering, enhancing the quality of its oil.     

Regulation of ornithine decarboxylase activity by amino acids, cyclic AMP and luteinizing hormone in cultured mammalian cells

Any one of five amino acis (alanine, asparagine, glutamine, glycine, and serine) is an essential requirement for the induction of ornithine decarboxylase (EC 4.1.1.17) in cultured chinese hamster ovary (CHO) cells maintained with a salts/glucose, medium. Each of these amino acids induced a striking activation of ornithine decarboxylase in the presence of dibutyryl cyclic AMP and luteinizing hormone. The effect of the other amino acids was considerably less or negligible. The active amino acids at optimal concentrations (10 mM) induced only a 10-20 fold enhancement of enzyme activity alone, while in the presence of dibutyryl cyclic AMP, ornithine decarboxylase activity was increased 40-50 fold within 7-8 h. Of the hormones and drugs tested, luteinizing hormone resulted in the highest (300-500 fold) induction of ornithine decarboxylase with optimal concentrations of dibutyryl cyclic AMP and asparagnine. Omission of dibutyryl cyclic AMP reduced this maximal activation to one half while optimal levels of luteinizing hormone alone caused no enhancement of ornithine decarboxylase activity. The induction of ornithine decarboxylase elicited by dibutyryl cyclic AMP, amino acid and luteinizing hormone was diminished about 50% with inhibitors of RNA and protein synthesis. The specific amino acid requirements for ornithine decarboxylase induction in chinese hamster ovary cells was similar to the requirements for induction in two other transformed cell lines. Understanding the mechanism of enzyme induction requires an identification of the essential components of the regulatory system. The essential requirement for enzyme induction is one of five amino acids. The induction of ornithine decarboxylase by dibutyryl cyclic AMP and luteinizing hormone was additive in the presence of an active amino acid.     

Differential translational effects of myristic acid and eicosapentaenoic acid on 3-hydroxy-3-methylglutaryl-CoA reductase from Reuber H35 hepatoma cells

The mechanisms by which saturated and polyunsaturated fatty acids may exert their effects on levels of blood cholesterol and human atherosclerosis have not been fully established. In this work, we studied the translational effects of myristic (14:0) and eicosapentaenoic (20:5) acids on 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase from Reuber H35 hepatoma cells. This enzyme is an intrinsic membrane, 96-kDa protein whose proteolysis releases an enzymatically active, 52- to 56-kDa, soluble fragment. We optimized an immunoblot procedure for quantifying small amounts of both the native and the soluble forms of HMG-CoA reductase from Reuber H35 hepatoma cells. We demonstrated that the upregulation of HMG-CoA reductase by a acid is due to an increase of the HMG-CoA reductase protein; therefore, protein synthesis would be required for the increase of HMG-CoA reductase activity caused by this fatty acid. In contrast, the downregulation of HMG-CoA reductase caused by eicosapentaenoic acid is not due to decreased protein synthesis, since similar levels of protein were found in the presence and absence of this fatty acid. Results obtained with cycloheximide as a protein-synthesis inhibitor confirm these findings.     

Multihormonal regulation of ornithine decarboxylase activity in highly differentiated porcine granulosa cells in vitro

Purified luteinizing hormone, but not follicle-stimulating hormone, elicited time- and dose-dependent stimulation of the cytosolic enzyme, ornithine decarboxylase, in highly differentiated, porcine granulosa cells maintained in vitro in chemically defined medium. Enzymic induction was susceptible to inhibitors of protein and RNA synthesis, and was suppressed by selective direct and indirect inhibitors of ornithine decarboxylase. Physiologic concentrations of prostaglandin E₂ and L-epinephrine also enhanced enzymic activity in a dose-dependent and saturable manner. Systematic comparison of the hormonal induction of ornithine decarboxylase in highly differentiated versus poorly differentiated granulosa cells revealed distinctive patterns of enzymic responsivity in relation to the degree of cytodifferentiation attained in vivo. This in vitro model is likely to permit further detailed examination of the molecular mechanisms subserving the hormonal control of ovarian ornithine decarboxylase activity in spontaneously differentiated granulosa cells maintained under chemically defined conditions in vitro.     

Amino acid requirement for the growth-hormone stimulation of incorporation of precursors into protein and nucleic acids of liver slices

1. Incorporation of [(14)C]leucine into protein in rat liver slices, incubated in vitro, increased as the concentration of unlabelled amino acids in the incubation medium was raised. A plateau of incorporation was reached when the amino acid concentration was 6 times that present in rat plasma. Labelling of RNA by [(3)H]orotic acid was not stimulated by increased amino acid concentration in the incubation medium. 2. When amino acids were absent from the medium, or present at the normal plasma concentrations, no effect of added growth hormone on labelling of protein or RNA by precursor was observed. 3. When amino acids were present in the medium at 6 times the normal plasma concentrations addition of growth hormone stimulated incorporation of the appropriate labelled precursor into protein of liver slices from normal rats by 31%, and into RNA by 22%. A significant effect was seen at a hormone concentration as low as 10ng/ml. 4. Under the same conditions addition of growth hormone also stimulated protein labelling in liver slices from hypophysectomized rats. Tissue from hypophysectomized rats previously treated with growth hormone did not respond to growth hormone in vitro. 5. No effect of the hormone on the rate or extent of uptake of radioactive precursors into acid-soluble pools was found. 6. Cycloheximide completely abolished the hormone-induced increment in labelling of both RNA and protein. 7. It was concluded that, in the presence of an abundant amino acid supply, growth hormone can stimulate the synthesis of protein in rat liver slices by a mechanism that is more sensitive to cycloheximide than is the basal protein synthesis. The stimulation of RNA labelling observed in the presence of growth hormone may be a secondary consequence of the hormonal effect on protein synthesis. 8. The mechanism of action of growth hormone on liver protein synthesis in vitro was concluded to be similar to its mechanism of action in vivo.     

Mechanism of fatty acid synthesis

Significant advances have been made in the past few years in our understanding of the mechanism of synthesis of fatty acids, the structural organization of fatty acid synthetase complexes and the mechanism of regulation of activity of these enzyme systems. Numerous fatty acid synthetase complexes have been purified to homogeneity and the mechanism of synthesis of fatty acids by these enzyme systems has been ascertained from tracer, and recently, kinetic studies. The results obtained by these methods are in complete agreement. Furthermore, the kinetic results have indicated that fatty acid synthesis proceeds by a seven-site ping-pong mechanism. Several of the fatty acid synthetases have been dissociated completely to nonidentical half-molecular weight subunit species and these have been separated by affinity chromatography. From one of these subunits acyl carrier protein has been obtained. Whether the nonidentical subunits can be dissociated into individual proteins or whether these subunits are each comprised of one peptide is still a matter of controversy. However, it appears to us that each of the half-molecular weight subunits is indeed comprised of individual proteins. Studies on the regulation of activity of fatty acid synthetase complexes of avian and mammalian liver have resulted in the separation by affinity chromatography of three species (apo, holo-$\text{a}$ and holo-$\text{b}$) of fatty acid synthetase. Since these species have radically different enzyme activities they may provide a mechanism of short-term regulation of fatty acid synthetase activity. Other studies have shown that the quantity of avian and mammalian liver fatty acid synthetases is controlled by a change in the rate of synthesis of this enzyme complex. This change in the rate of synthesis of enzyme complex is under the control of insulin and glucagon. The former hormone increases the rate of enzyme synthesis, whereas the latter decreases it. Further studies on fatty acid synthetase complexes will undoubtedly concentrate upon more refined aspects of the structural organization of these enzyme systems, including the sequencing of acyl carrier proteins, the effects of protein-protein interaction on the kinetics of the partial reactions of fatty acid synthesis catalyzed by separated enzymes of the complex, the mechanism of hormonal regulation of fatty acid synthetase activity and x-ray diffraction analysis of subunits and complex.     

Arachidonic acid potentiates exocytosis and allows neuronal SNARE complex to interact with Munc18a

Neuronal communication relies on the fusion of neurotransmitter-containing vesicles with the neuronal plasma membrane. Recent genetic studies have highlighted the critical role played by polyunsaturated fatty acids in neurotransmission, however, there is little information available about which fatty acids act on exocytosis and, more importantly, by what mechanism. We have used permeabilized chromaffin cells to screen various fatty acids of the n-3 and n-6 series for their acute effects on exocytosis. We have demonstrated that an n-6 series polyunsaturated fatty acid, arachidonic acid, potentiates secretion from intact neurosecretory cells regardless of the secretagogue used. We have shown that arachidonic acid dose dependently increases soluble NSF attachment protein receptor complex formation in chromaffin cells and bovine cortical brain extracts and that a non-hydrolysable analogue of arachidonic acid causes a similar increase in SNARE complex formation. This prompted us to examine the effect of arachidonic acid on SNARE protein interactions with Munc18a, a protein known to prevent Syntaxin1a engagement into the SNARE complex in vitro. In the presence of arachidonic acid, we show that Munc18a can interact with the neuronal SNARE complex in a dose-dependent manner. We further demonstrate that arachidonic acid directly interacts with Syntaxin1a.     

Effects of inhibitors of protein and RNA synthesis on aldosterone-stimulated changes in phospholipid fatty acid metabolism in the toad urinary bladder.

The effects of an inhibitor of RNA synthesis, cordycepin, and an inhibitor of protein synthesis, cycloheximide, on aldosterone-induced changes in lipid metabolism and phospholipid fatty acid composition have been studied in the toad urinary bladder. At the concentrations employed, the inhibitors abolish the hormone-induced increases in total lipid synthesis, phospholipid fatty acid specific activities, and weight percentage of phospholipid long-chain polyunsaturated fatty acids as well as blocking the aldosterone-mediated increase in sodium transport.     

In vivo effects of follicle-stimulating hormone on testicular macrophages

The effect of an intraperitoneal injection of various hormones on the incorporation of amino acids and uridine into acid-precipitable material by subsequently isolated testicular macrophages was investigated. It was found that treatment with follicle-stimulating hormone (FSH), but not luteinizing hormone (LH) or insulin, significantly increased incorporation of amino acids into secreted (but not cellular) protein and uridine incorporation into cellular RNA in a dose-dependent manner. Maximal responsiveness was observed at a dose of 50 micrograms of the hormone. These studies demonstrate that FSH has an action on testicular macrophages in vivo.     

Addition of an N-terminal epitope tag significantly increases the activity of plant fatty acid desaturases expressed in yeast cells

Saccharomyces cerevisiae shows great potential for development of bioreactor systems geared toward the production of high-value lipids such as polyunsaturated omega-3 fatty acids, the yields of which are largely dependent on the activity of ectopically expressed enzymes. Here, we show that the addition of an N-terminal epitope tag sequence (either Myc or hemagglutinin) to oleate desaturase (FAD2) or omega-3 linoleate desaturase (FAD3) enzymes from plants, which catalyze consecutive reactions in the production of long chain omega-3 fatty acids, significantly increases their activity up to fourfold when expressed in yeast cells. Quantitative protein blotting using an antibody specific for native FAD2 revealed that the steady-state amount of the epitope-tagged FAD2 protein was also approximately fourfold higher than that of its untagged counterpart, demonstrating a direct relationship between the epitope tag-induced increase in enzyme amount and fatty acid product formation. Protein half-life and RNA blotting experiments indicated that the half-lives and mRNA content of the tagged and untagged FAD2 proteins were essentially the same, suggesting that the epitope tags increased protein abundance by improving translational efficiency. Taken together, these results indicate that the addition of an epitope tag sequence to a plant fatty acid desaturase (FAD) not only provides a useful means for protein immunodetection using highly specific, commercially available antibodies, but that it also significantly increases FAD activity and the production of polyunsaturated fatty acids in yeast cells.     

Mutation of host delta9 fatty acid desaturase inhibits brome mosaic virus RNA replication between template recognition and RNA synthesis

All positive-strand RNA viruses assemble their RNA replication complexes on intracellular membranes. Brome mosaic virus (BMV) replicates its RNA in endoplasmic reticulum (ER)-associated complexes in plant cells and the yeast Saccharomyces cerevisiae. BMV encodes RNA replication factors 1a, with domains implicated in RNA capping and helicase functions, and 2a, with a central polymerase-like domain. Factor 1a interacts independently with the ER membrane, viral RNA templates, and factor 2a to form RNA replication complexes on the perinuclear ER. We show that BMV RNA replication is severely inhibited by a mutation in OLE1, an essential yeast chromosomal gene encoding delta9 fatty acid desaturase, an integral ER membrane protein and the first enzyme in unsaturated fatty acid synthesis. OLE1 deletion and medium supplementation show that BMV RNA replication requires unsaturated fatty acids, not the Ole1 protein, and that viral RNA replication is much more sensitive than yeast growth to reduced unsaturated fatty acid levels. In ole1 mutant yeast, 1a still becomes membrane associated, recruits 2a to the membrane, and recognizes and stabilizes viral RNA templates normally. However, RNA replication is blocked prior to initiation of negative-strand RNA synthesis. The results show that viral RNA synthesis is highly sensitive to lipid composition and suggest that proper membrane fluidity or plasticity is essential for an early step in RNA replication. The strong unsaturated fatty acid dependence also demonstrates that modulating fatty acid balance can be an effective antiviral strategy.     

The gonadotropin-releasing hormone receptor: signals involved in gonadotropin secretion and biosynthesis

The neurohormone gonadotropin-releasing hormone (GnRH) is a decapeptide which is synthesized in the hypothalamus and released into the hypophysial portal system in a pulsatile manner. GnRH exerts its effect on the anterior pituitary gonadotrophs where it regulates the secretion and synthesis of gonadotropins (luteinizing hormone and follicle-stimulating hormone) through receptor-mediated actions. The GnRH receptor has been characterized and shown to be coupled to the formation of 'second messengers' which participate in signal transduction mechanisms. GnRH stimulation of luteinizing hormone release is a Ca2(+)-dependent process. G protein, phosphoinositide hydrolysis, protein kinase C as well as arachidonic acid and some of its metabolites were identified as possible mediators in the process.     

Coupling of fatty acid and phospholipid synthesis in Bacillus subtilis

plsX (acyl-acyl carrier protein [ACP]:phosphate acyltransferase), plsY (yneS) (acyl-phosphate:glycerol-phosphate acyltransferase), and plsC (yhdO) (acyl-ACP:1-acylglycerol-phosphate acyltransferase) function in phosphatidic acid formation, the precursor to membrane phospholipids. The physiological functions of these genes was inferred from their in vitro biochemical activities, and this study investigated their roles in gram-positive phospholipid metabolism through the analysis of conditional knockout strains in the Bacillus subtilis model system. The depletion of PlsX led to the cessation of both fatty acid synthesis and phospholipid synthesis. The inactivation of PlsY also blocked phospholipid synthesis, but fatty acid formation continued due to the appearance of acylphosphate intermediates and fatty acids arising from their hydrolysis. Phospholipid synthesis ceased following PlsC depletion, but fatty acid synthesis continued at a high rate, leading to the accumulation of fatty acids arising from the dephosphorylation of 1-acylglycerol-3-P followed by the deacylation of monoacylglycerol. Analysis of glycerol 3-P acylation in B. subtilis membranes showed that PlsY was an acylphosphate-specific acyltransferase, whereas PlsC used only acyl-ACP as an acyl donor. PlsX was found in the soluble fraction of disrupted cells but was associated with the cell membrane in intact organisms. These data establish that PlsX is a key enzyme that coordinates the production of fatty acids and membrane phospholipids in B. subtilis.     

Fatty acid metabolism in Paramecium. Oleic acid metabolism and inhibition of polyunsaturated fatty acid synthesis by triparanol

Paramecium requires oleic acid for growth and can grow in media containing no other fatty acids. In the present study, we have shown that this ciliate utilized oleate mainly as a carbon and energy source, even though this fatty acid was the only substrate available for synthesis of polyunsaturated fatty acids. Culture growth was inhibited by the addition of the drug triparanol. Triparanol decreased the formation of polyunsaturated fatty acids from oleate by preventing desaturation to form the dienoic acid, linoleate. Triparanol inhibition resulted in an altered phospholipid fatty acyl composition, an increased fragility and an altered behavioral response of the cells to a depolarizing stimulation solution. Therefore, although most of the dietary oleate was not used by the cells for polyunsaturated fatty acid synthesis, the desaturation of oleic acid was critical for normal culture growth, cell integrity and swimming behavior, all of which are expected to be dependent on normal membrane lipid composition.     

Ribonucleic acid synthesis in rat liver during fatty acid stimulated secretion of very low density lipoproteins.

Production of very low density lipoproteins by the liver depends on the cellular availability of fatty acids. It is stimulated by the uptake of free fatty acids from the plasma and by increased lipogenesis and is inhibited by actinomycin D, suggesting that RNA synthesis is involved in the regulation of apolipoprotein synthesis. This hypothesis has been investigated in rats in vivo and in isolated perfused livers with and without stimulation by fatty acid overload: [14C] orotate incorporation in liver polyribosomal RNA is 60 per cent greater in stimulated livers as compared to controls. This increase is primarily due to a higher incorporation in bound polysomes and in those containing at least six ribosomes and does not result from the inhibition of ribonuclease. RNase digestion of polysomal RNA (4.10(-10) M enzyme, 0 degrees C, 3 h) shows that there is twice as much radioactivity in the hydrolyzed RNA of stimulated livers as compared to controls. After partial purification of poly A-rich RNA by affinity chromatography, the mass yield and radioactivity are increased by 100 per cent in stimulated livers as compared to controls. In conclusion, de novo RNA synthesis seems to be necessary for fatty acid stimulation of VLDL production.     

Expression of an ovine growth hormone transgene in mice increases archidonic acid in cellular membranes

The degree of unsaturation of membrane lipids has been implicated in a number of physiological disorders, yet its regulation remains poorly understood, especially the regulation of the synthesis and distribution of arachidonic acid levels, the most abundant long chain polyunsaturated fatty acid in membranes. Transgenic mice expressing the ovine metallothionein 1a — ovine growth hormone (oMt1a-oGH) fusion gene exhibited significantly elevated levels of a number of long chain polyusaturated fatty acids in serum, including arachidonic acid. In oMt1a-oGH transgenic mice the products of all three desaturation pathways are affected by the expression of the ovine growth hormone trangene. The essential precursors of membrane long chain polyunsturated fatty acids, 18:2n-6 and 18:3n-3, were reduced in transgenic relative to controls, and their desaturation and elongation products, arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3), were elevated. As rare intermediate long chain polyunsaturated fatty acids such as eicosatrienoic acid (20:3n-9) were also signficantly elevated, we conclude that these observations reflect increased activity of the Δ-5 and Δ-6 desaturase enzymes. In contranst, the products of the stearoyl CoA or Δ-9 desaturase, were significantly reduced in oMt1a-oGH expressing transgenics relative to their levels in control mice.