Stimulation of yeast adenylyl cyclase activity by lysophospholipids and fatty acids. Implications for the regulation of Ras/effector function by lipids

Seipin: harvesting fat and keeping adipocytes healthy

Seipin is a key protein in the assembly of cytoplasmic lipid droplets (cLDs) and their maintenance at endoplasmic reticulum (ER)-LD junctions; the absence of seipin results in generalized lipodystrophy. How seipin mediates LD dynamics and prevents lipodystrophy are not well understood. New evidence suggests that seipin attracts triglyceride monomers from the ER to sites of droplet formation. By contrast, seipin may not be directly involved in the assembly of nuclear LDs and may actually suppress their formation at a distance. Seipin promotes adipogenesis, but lipodystrophy may also involve postadipogenic effects. We hypothesize that among these are a cycle of runaway lipolysis and lipotoxicity caused by aberrant LDs, resulting in a depletion of fat stores and a failure of adipose and other cells to thrive.

On the potential of mass spectrometry-based metabolite profiling approaches to the study of biochemical adaptation in psychrophilic yeast

To move beyond targeted approaches to the biochemical characterization of psychrophilic yeast and provide a more holistic understanding of the chemistry of physiological adaptation of psychrophiles at the molecular level, ultraperformance liquid chromatography combined with simultaneous acquisition of low- and high-collision energy mass spectra (UPLC/MS(e)) was employed for a preliminary comparative analysis of cell extracts of psychrophilic Antarctic yeasts Cryptococcus vishniacii CBS 10616 and Dioszegia cryoxerica CBS 10919 versus the mesophile Saccharomyces cerevisiae 'cry havoc'. A detailed workflow for providing high-confidence preliminary identifications of psychrophilic yeast-specific molecular features is presented. Preliminary identifications of psychrophile-specific features in C. vishniacii and D. cryoxerica determined with the described method include the glycerophospholipids lysophosphatidylcholine 18:2, lysophosphatidylcholine 18:3, lysophosphatidylethanolamine 18:3, and lysophosphatidylethanolamine 18:2. In addition, levels of guanosine diphosphate appear significantly elevated in cell extracts of the psychrophilic yeasts as compared to Saccharomyces cerevisiae. Finally, five psychrophilic yeast-specific peptides have been discovered. All of these are demonstrated to be glycine- and/or proline-rich, a known structural characteristic of many naturally occurring bioactive peptides. The potential of this untargeted metabolite profiling approach as a tool for knowledge discovery and hypothesis generation in the study of biodiversity and microbial adaptation is highlighted.

Oleate inhibits steryl ester synthesis and causes liposensitivity in yeast

In the yeast Saccharomyces cerevisiae, neutral lipids can be synthesized by four acyltransferases, namely Dga1p and Lro1p producing triacylglycerols (TAG) and Are1p and Are2p forming steryl esters (SE). TAG and SE are stored in an organelle called lipid particles/droplet. Growth of yeast cells on oleate-supplemented media strongly induced proliferation of lipid particles and specifically the synthesis of TAG, which serve as the major pool for the excess of fatty acids. Surprisingly, SE synthesis was strongly inhibited under these conditions. Here, we show that this effect was not due to decreased expression of ARE2 encoding the major yeast SE synthase at the transcriptional level but to competitive enzymatic inhibition of Are2p by free oleate. Consequently, a triple mutant dga1Deltalro1Deltaare1DeltaARE2(+) grown on oleate did not form substantial amounts of SE and exhibited a growth phenotype similar to the dga1Deltalro1Deltaare1Deltaare2Delta quadruple mutant, including lack of lipid particles. Growth of these mutants on oleate was strongly delayed, and cell viability was decreased but rescued by adaptation. In these strains, oleate stress caused morphological changes of intracellular membranes, altered phospholipid composition and formation of an additional lipid class, ethyl esters of fatty acids. In summary, our data showed that exposure to oleate led to disturbed lipid and membrane homeostasis along with liposensitivity of the yeast.

Cytosolic phospholipase A2: a member of the signalling pathway of a new G protein alpha subunit in Sporothrix schenckii

Background

Sporothrix schenckii is a pathogenic dimorphic fungus, the etiological agent of sporotrichosis, a lymphocutaneous disease that can remain localized or can disseminate, involving joints, lungs, and the central nervous system. Pathogenic fungi use signal transduction pathways to rapidly adapt to changing environmental conditions and S. schenckii is no exception. S. schenckii yeast cells, either proliferate (yeast cell cycle) or engage in a developmental program that includes proliferation accompanied by morphogenesis (yeast to mycelium transition) depending on the environmental conditions. The principal intracellular receptors of environmental signals are the heterotrimeric G proteins, suggesting their involvement in fungal dimorphism and pathogenicity. Identifying these G proteins in fungi and their involvement in protein-protein interactions will help determine their role in signal transduction pathways.

Results

In this work we describe a new G protein α subunit gene in S. schenckii, ssg-2. The cDNA sequence of ssg-2 revealed a predicted open reading frame of 1,065 nucleotides encoding a 355 amino acids protein with a molecular weight of 40.9 kDa. When used as bait in a yeast two-hybrid assay, a cytoplasmic phospholipase A₂ catalytic subunit was identified as interacting with SSG-2. The sspla₂ gene, revealed an open reading frame of 2538 bp and encoded an 846 amino acid protein with a calculated molecular weight of 92.62 kDa. The principal features that characterize cPLA₂ were identified in this enzyme such as a phospholipase catalytic domain and the characteristic invariable arginine and serine residues. A role for SSPLA₂ in the control of dimorphism in S. schenckii is suggested by observing the effects of inhibitors of the enzyme on the yeast cell cycle and the yeast to mycelium transition in this fungus. Phospholipase A₂ inhibitors such as AACOCF3 (an analogue of archidonic acid) and isotetrandrine (an inhibitor of G protein PLA₂ interactions) were found to inhibit budding by yeasts induced to re-enter the yeast cell cycle and to stimulate the yeast to mycelium transition showing that this enzyme is necessary for the yeast cell cycle.

Conclusion

A new G protein α subunit gene was characterized in S. schenckii and protein-protein interactions studies revealed this G protein alpha subunit interacts with a cPLA₂ homologue. The PLA₂ homologue reported here is the first phospholipase identified in S. schenckii and the first time a PLA₂ homologue is identified as interacting with a G protein α subunit in a pathogenic dimorphic fungus, establishing a relationship between these G proteins and the pathogenic potential of fungi. This cPLA₂ homologue is known to play a role in signal transduction and fungal pathogenesis. Using cPLA₂ inhibitors, this enzyme was found to affect dimorphism in S. schenckii and was found to be necessary for the development of the yeast or pathogenic form of the fungus.

Identification of a novel lysophospholipid acyltransferase in Saccharomyces cerevisiae

The incorporation of unsaturated acyl chains into phospholipids during de novo synthesis is primarily mediated by the 1-acyl-sn-glycerol-3-phosphate acyltransferase reaction. In Saccharomyces cerevisiae, Slc1 has been shown to mediate this reaction, but distinct activity remains after its removal from the genome. To identify the enzyme that mediates the remaining activity, we performed synthetic genetic array analysis using a slc1Delta strain. One of the genes identified by the screen, LPT1, was found to encode for an acyltransferase that uses a variety of lysophospholipid species, including 1-acyl-sn-glycerol-3-phosphate. Deletion of LPT1 had a minimal effect on 1-acyl-sn-glycerol-3-phosphate acyltransferase activity, but overexpression increased activity 7-fold. Deletion of LPT1 abrogated the esterification of other lysophospholipids, and overexpression increased lysophosphatidylcholine acyltransferase activity 7-fold. The majority of this activity co-purified with microsomes. To test the putative role for this enzyme in selectively incorporating unsaturated acyl chains into phospholipids in vitro, substrate concentration series experiments were performed with the four acyl-CoA species commonly found in yeast. Although the saturated palmitoyl-CoA and stearoyl-CoA showed a lower apparent Km, the monounsaturated palmitoleoyl-CoA and oleoyl-CoA showed a higher apparent Vmax. Arachidonyl-CoA, although not abundant in yeast, also had a high apparent Vmax. Pulse-labeling of lpt1Delta strains showed a 30% reduction in [3H]oleate incorporation into phosphatidylcholine only. Therefore, Lpt1p, a member of the membrane-bound o-acyltransferase gene family, seems to work in conjunction with Slc1 to mediate the incorporation of unsaturated acyl chains into the sn-2 position of phospholipids.

Phospholipids: synthesis, sorting, subcellular traffic - the yeast approach

Most of the enzymes and genes required for lipid biosynthesis and degradation in the budding yeast Saccharomyces cerevisiae have now been identified and the global mechanisms that regulate their activity are being established. Synthesis of phospholipids is restricted to specific subcellular compartments, and the lipids migrate from their site of formation to their final destination. In addition to synthesis, remodelling and degradation of phospholipids controls the content of the lipid portion of cellular membranes, while highly specific phospholipases catalyse the release of lipid-based second messengers. In this review, we describe the current understanding of the organization and regulation of phospholipid metabolism in yeast, and discuss the mechanisms that have been proposed for intracellular lipid transport.

Inhibition of adenylylcyclase activity in mouse cerebellum membranes upon hydrolysis of triacylglycerols by triacylglycerol lipase, but not phospholipids by phospholipase A(2)

We previously showed that arachidonic acid and related unsaturated free fatty acids (U-FFAs) inhibit the activity of adenylylcyclase in brain membranes of mice. The level of U-FFAs elevates when the hydrolysis of triacylglycerols (TAGs) and phospholipids is promoted. In this study, we examined whether activation of triacylglycerol lipase (TAG lipase) and phospholipase A(2) (PLA(2)) results in the inhibition of adenylylcyclase activity in cerebellum membranes of mice. Incubation of Intralipos with TAG lipase in the presence of membranes mainly released oleic acid and linoleic acid and caused > or =95% inhibition of adenylylcyclase activity. In contrast, PLA(2), though releasing substantial amounts of U-FFAs, increased the enzymatic activity. To account for this difference, we examined how by-products formed in U-FFA release by TAG lipase and PLA(2) operated on the arachidonic acid-induced inhibition. Lysophosphatidylcholne and some other lysophospholipids, produced by PLA(2), enhanced the adenylylcyclase activity and attenuated the inhibitory effect of arachidonic acid. On the other hand, no such effects were found with by-products of TAG lipase-mediated lipolysis. Rather, monoacylglycerols having U-FFAs, possibly formed by TAG lipase, potentiated the arachidonic acid-induced inhibition of adenylylcyclase. Bovine serum albumin, added into the mixture for the pretreatment of membranes with TAG lipase, prevented the inhibition of adenylylcyclase. These results indicate that by-products formed in U-FFA release have a crucial role for the U-FFA's action on adenylylcyclase and that U-FFAs released from TAG are an inhibitor of adenylylcyclase. It may be that albumin in plasma, and thus FFA-binding proteins within cells, are of importance in protecting adenylylcyclase upon U-FFA release.

Inhibition of adenylyl cyclase activity in brain membrane fractions by arachidonic acid and related unsaturated fatty acids

Pretreatment of mouse brain membranes with arachidonic acid (AA) and related unsaturated fatty acids at 30 degrees C for 10 min decreased basal activity and isoproterenol/guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)- and forskolin-stimulated activities of adenylyl cyclase to a level less than 5% of control. The presence of the carboxyl group on the fatty acids was essential for the inhibition, because no such inhibition was found with ethyl arachidonate or AA attached to diacylglycerols and phospholipids. The AA-mediated inhibition was observed when the activity was measured in the presence of Mn2+ or forskolin and was insensitive to pertussis toxin or guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), indicating a mechanism independent of GTP-binding proteins. In addition, the fact that stimulators of the adenylyl cyclase catalytic unit, ATP, GTP gamma S and forskolin, when present during pretreatment, attenuate the inhibitory effect of AA may suggest that the catalytic unit is a target of AA. Bovine serum albumin suppressed the inhibition when present in the mixtures for pretreatment, but could not restore the adenylyl cyclase activity that had been reduced by AA, indicating an irreversible inhibition by AA. The effect of AA was found to be additive to P-site-mediated inhibition. The present study suggests the existence of another mechanism of regulation of adenylyl cyclase by unsaturated fatty acids.

Geranylgeranylated SNAREs are dominant inhibitors of membrane fusion

Exocytosis in yeast requires the assembly of the secretory vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (v-SNARE) Sncp and the plasma membrane t-SNAREs Ssop and Sec9p into a SNARE complex. High-level expression of mutant Snc1 or Sso2 proteins that have a COOH-terminal geranylgeranylation signal instead of a transmembrane domain inhibits exocytosis at a stage after vesicle docking. The mutant SNARE proteins are membrane associated, correctly targeted, assemble into SNARE complexes, and do not interfere with the incorporation of wild-type SNARE proteins into complexes. Mutant SNARE complexes recruit GFP-Sec1p to sites of exocytosis and can be disassembled by the Sec18p ATPase. Heterotrimeric SNARE complexes assembled from both wild-type and mutant SNAREs are present in heterogeneous higher-order complexes containing Sec1p that sediment at greater than 20S. Based on a structural analogy between geranylgeranylated SNAREs and the GPI-HA mutant influenza virus fusion protein, we propose that the mutant SNAREs are fusion proteins unable to catalyze fusion of the distal leaflets of the secretory vesicle and plasma membrane. In support of this model, the inverted cone-shaped lipid lysophosphatidylcholine rescues secretion from SNARE mutant cells.

Recombinant maize protoporphyrinogen IX oxidase expressed in Escherichia coli forms complexes with GroEL and DnaK chaperones

The clone corresponding to maize plastidic protoporphyrinogen IX oxidase (PPO) has been isolated by functional complementation and inserted into a pET16b vector for expression in Escherichia coli. Recombinant PPO was purified by standard affinity chromatography using a metal chelating resin. Two contaminants copurified with recombinant PPO and were identified as GroEL and DnaK. Since chaperone binding to hydrophobic regions of the protein is regulated by ATP availability, an ATP washing step was introduced prior to elution of the recombinant protein from an affinity column. This washing step selectively removed both chaperones and allowed the recovery of pure PPO. Coexpression of PPO and GroELS resulted in a sixfold increase of soluble PPO yield, suggesting that bacterial chaperones could be limiting during the folding of the heterologous protein. However, a portion of PPO was still found in the insoluble fraction. Buffer containing the GroEL and DnaK enabled resuspension of PPO from the insoluble fraction but failed to enhance refolding of the denaturated protein. Attempts to increase the amount of soluble PPO using a thioredoxin-PPO fusion protein were not successful. Initial characterization of the recombinant PPO found that it possessed a high V(max), an elevated affinity for substrate, and an elevated sensitivity to PPO inhibitor herbicides compared to previous reports.

Lysophosphatidylcholine stimulates the release of arachidonic acid in human endothelial cells

Lysophosphatidylcholine (lyso-PC) is a product of phosphatidylcholine hydrolysis by phospholipase A2 (PLA2) and is present in cell membranes, oxidized lipoproteins, and atherosclerotic tissues. It has the ability to alter endothelial functions and is regarded as a causal agent in atherogenesis. In this study, the modulation of arachidonate release by lyso-PC in human umbilical vein endothelial cells was examined. Incubation of endothelial cells with lyso-PC resulted in an enhanced release of arachidonate in a time- and concentration-dependent manner. Maximum arachidonate release was observed at 10 min of incubation with 50 microM lyso-PC. Lyso-PC species containing palmitoyl (C16:0) or stearoyl (C18:0) groups elicited the enhancement of arachidonate release, while other lysolipids such as lysophosphatidylethanolamine, lysophosphatidylserine, lysophosphatidylinositol, or lysophosphatidate were relatively ineffective. Lyso-PC-induced arachidonate release was decreased by treatment of cells with PLA2 inhibitors such as para-bromophenacyl bromide and arachidonoyl trifluoromethyl ketone. Furthermore, arachidonate release was attenuated in cells grown in the presence of antisense oligodeoxynucleotides that specifically bind cytosolic PLA2 mRNA. Treatment of cells with lyso-PC resulted in a translocation of PLA2 activity from the cytosolic to the membrane fractions of cells. Lyso-PC induced a rapid influx of Ca2+ from the medium into the cells, with a simultaneous enhancement of protein kinase C (PKC) activity in the membrane fractions. The lyso-PC-induced arachidonate release was attenuated when cells were preincubated with specific inhibitors of PKC (staurosporine and Ro31-8220) or a specific inhibitor of mitogen-activated protein kinase/extracellular regulated kinase kinase (PD098059). Taken together, the results of this study show that lyso-PC caused the elevation of cellular Ca2+ and the activation of PKC, which stimulated cytosolic PLA2 in an indirect manner and resulted in an enhanced release of arachidonate.

The bioactive phospholipid, lysophosphatidylcholine, induces cellular effects via G-protein-dependent activation of adenylyl cyclase

The naturally occurring phospholipid, lysophosphatidylcholine (lyso-PC), regulates a broad range of cell processes, including gene transcription, mitogenesis, monocyte chemotaxis, smooth muscle relaxation, and platelet activation. Despite the growing list of cellular effects attributable to lyso-PC, the mechanism(s) by which it alters cell function have not been elucidated. In this report, we have examined the effects of exogenous lyso-PC on signal transduction processes within a variety of lyso-PC-responsive cells, including human platelets, monocyte-like THP-1 cells, and the megakaryoblastic cell line, MEG-01. Pretreatment of each of these cells with increasing concentrations of lyso-PC (25-150 microg/ml) was associated with a progressive increase in the cytosolic concentration of cAMP. The accumulation of cAMP in platelets correlated closely with the ability of lyso-PC to inhibit multiple platelet processes, including platelet aggregation, agonist-induced protein kinase C activation, thromboxane A2 generation, and the tyrosine phosphorylation of platelet proteins. In each of the cell types examined, the ability of lyso-PC to increase the cellular levels of cAMP was synergistically enhanced by pretreating the cells with the cAMP phosphodiesterase inhibitor, theophylline (5 mM), and was specifically inhibited by the P-site inhibitor of adenylyl cyclase, 2,5-dideoxyadenosine. A role for the stimulatory G-protein, Gs, in the lyso-PC-induced activation of adenylyl cyclase was suggested by the ability of the GTPase inhibitor, guanylyl 5'-thiophosphate (0.2 mM), to inhibit the lyso-PC-stimulated increase in cAMP, and also by the ability of cholera toxin to inhibit increases in membrane GTPase activity in response to lyso-PC. The functional significance of lyso-PC-induced activation of adenylyl cyclase was investigated in MEG-01 cells. Treatment of these cells with either lyso-PC or dibutyryl cAMP for 36-40 h resulted in a 3-5-fold increase in the surface expression of the natural anticoagulant protein, thrombomodulin (TM). The ability of lyso-PC to increase TM expression was abolished by pretreating these cells with the adenylyl cyclase inhibitor, 2,5-dideoxyadenosine, whereas the dibutyryl cAMP-induced increase in TM remained insensitive to adenylyl cyclase inhibition. These studies define an important role for the adenylyl cyclase signaling system in mediating cellular effects induced by lyso-PC.

Interactions of protein kinase C with insulin signaling. Influence on GAP and Sos activities

In this study, we investigated the influence of the protein kinase C (PKC)-dependent system upon the ability of insulin to stimulate p21(ras).GTP loading in 3T3-L1 adipocytes. Activation of PKC by 12-0-tetradecanoylphorbol-13-acetate (TPA) did not affect the basal amount of p21(ras).GTP but significantly reduced insulin-induced increases in p21(ras).GTP. This reduction was due to inhibition of the insulin's ability to stimulate guanine nucleotide exchange activity of Sos in cells incubated with 100 nM TPA for either 30 min or 3 h. TPA had no effect on basal activity of Sos. Depletion of PKC by an 18-h incubation with TPA or inhibition by bisindolylmaleimide resulted in profound inhibition of the insulin-induced p21(ras).GTP loading. In contrast to PKC activation, removal of PKC did not influence Sos activity but resulted in a 2-fold stimulation of GTPase activating protein (GAP). This effect of PKC depletion is unique to 3T3-L1 adipocytes and was not observed in either 3T3-L1 fibroblasts or Rat-1 fibroblasts. Thus, it appears that in 3T3-L1 adipocytes, PKC has a constitutive inhibitory effect on GAP that permits insulin to activate Sos and p21(ras). Removal of this inhibitory influence activates GAP and reduces insulin-stimulated p21(ras).GTP loading.

Transcriptional induction of endothelial nitric oxide synthase type III by lysophosphatidylcholine

Endothelial synthesis of NO is catalyzed by constitutive NO synthase type III (NOS-III). NOS-III has been thought to be regulated mainly at the level of enzyme activity by intracellular calcium. We report that in human umbilical vein endothelial cells lysophosphatidylcholine (lyso-PC), a component of atherogenic lipoproteins and atherosclerotic lesions, increases NOS-III mRNA and protein levels. This leads to the augmentation of NOS-III activity and the enhancement of antiplatelet properties of endothelial cells. Importantly, nuclear run-off experiments demonstrate a transcriptional mechanism of regulation of NOS-III expression by lysophosphatidylcholine. As endothelium-derived NO appears to be an anti-atherogenic molecule, induction of NOS-III by lyso-PC may be a protective response that limits the progress of the atherosclerotic lesion and promotes its regression.