Metal ion stimulation of phospholipase D-like activity of isolated rat intestinal mitochondria

Lipid signaling on the mitochondrial surface

Regulated production and elimination of the signaling lipids phosphatidic acid (PA), diacylglycerol (DAG), and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) creates a complex and interconnected signaling network that modulates a wide variety of eukaryotic cell biological events. PA production at the plasma membrane and on trafficking membrane organelles by classical Phospholipase D (PLD) through the hydrolysis of phosphatidylcholine (PC) has been studied widely. In this chapter, we review a newly identified, non-canonical member of the PLD superfamily, MitoPLD, which localizes to the mitochondrial surface and plays a role in mitochondrial fusion via the hydrolysis of cardiolipin (CL) to generate PA. The role of PA in facilitating the mitochondrial fusion event carried out by proteins known as Mitofusins is intriguing in light of the role classic PLD-generated PA plays in facilitating SNARE-mediated fusion of secretory membrane vesicles into the plasma membrane. In addition, however, PA on the mitochondrial surface may also trigger a signaling cascade that elevates DAG, leading to downstream events that affect mitochondrial fission and energy production. PA production on the mitochondrial surface may also stimulate local production of PI4,5P(2) to facilitate mitochondrial fission and subcellular trafficking or facilitate Ca(2+) influx.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Phosphohydrolase and transphosphatidylation reactions of two Streptomyces phospholipase D enzymes: covalent versus noncovalent catalysis

A kinetic comparison of the hydrolase and transferase activities of two bacterial phospholipase D (PLD) enzymes with little sequence homology provides insights into mechanistic differences and also the more general role of Ca(2+) in modulating PLD reactions. Although the two PLDs exhibit similar substrate specificity (phosphatidylcholine preferred), sensitivity to substrate aggregation or Ca(2+), and pH optima are quite distinct. Streptomyces sp. PMF PLD, a member of the PLD superfamily, generates both hydrolase and transferase products in parallel, consistent with a mechanism that proceeds through a covalent phosphatidylhistidyl intermediate where the rate-limiting step is formation of the covalent intermediate. For Streptomyces chromofuscus PLD, the two reactions exhibit different pH profiles, a result consistent with a mechanism likely to involve direct attack of water or an alcohol on the phosphorus. Ca(2+), not required for monomer or micelle hydrolysis, can activate both PLDs for hydrolysis of PC unilamellar vesicles. In the case of Streptomyces sp. PMF PLD, Ca(2+) relieves product inhibition by interactions with the phosphatidic acid (PA). A similar rate enhancement could occur with other HxKx(4)D-motif PLDs as well. For S. chromofuscus PLD, Ca(2+) is absolutely critical for binding of the enzyme to PC vesicles and for PA activation. That the Ca(2+)-PA activation involves a discreet site on the protein is suggested by the observation that the identity of the C-terminal residue in S. chromofuscus PLD can modulate the extent of product activation.

Molecular cloning of a phospholipase D gene from Aspergillus nidulans and characterization of its deletion mutants

We cloned a gene pldA encoding a protein containing phospholipase D (PLD) motifs from a filamentous fungus Aspergillus nidulans. The deduced protein product of pldA consists of 833 amino acids and contains four conserved regions of a PLD gene family. Deletion mutants of pldA grew and formed conidia in a normal manner. Although PLD and transphosphatidylation activities against phosphatidylcholine of the mutant cell extract did not change, the Ca(2+)-dependent PLD activity against phosphatidylethanolamine was significantly reduced, but not in the wild-type cell extract. This activity was markedly enhanced by high osmotic growth conditions in the wild-type cells, and pldA of A. nidulans likely encodes a Ca(2+)-dependent phosphatidylethanolamine-hydrolyzing PLD.

Aluminum accumulation and membrane fluidity alteration in synaptosomes isolated from rat brain cortex following aluminum ingestion: effect of cholesterol

In the present work, we studied the effect of cholesterol/phospholipid (CH/PL) molar ratio on aluminum accumulation and aluminum-induced alteration of membrane fluidity in rat brain cortex synaptosomes. We observed that sub-acute (daily supply of 1.00 g of AlCl(3) during 10 days) and chronic (daily supply of 0.03 g of AlCl(3) during 4 months) exposure to dietary aluminum leads to a synaptosomal aluminum enrichment of 45 and 59%, respectively. During chronic exposure to AlCl(3), the enhancement of aluminum content was prevented by administration of colestipol (0.31 g/day), which decreased the synaptosomal membrane CH/PL molar ratio (nmol/nmol) from 1.2 to 0.4. Fluorescence anisotropy analysis, using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-(trimethylamino)phenyl)-6-phenylhexa-1,3,5-triene (TMA-DPH), showed that after treatment with colestipol a decrease in membrane order occurs at the level of hydrophilic lipid-water surface and deeper hydrophobic region of the synaptosomal membrane. When the rats were exposed to aluminum, it was observed a significant enhancement of membrane fluidity, which was more pronounced at the level of the membrane hydrophilic regions. Meanwhile, when chronic exposure to dietary AlCl(3) was accompanied by treatment with colestipol, the aluminum-induced decrease in membrane order was negligible when compared to TMA-DPH and DPH anisotropy values measured upon colestipol treatment. In contrast, in vitro incubation of synaptosomes (isolated from control rats) with AlCl(3) induced a concentration-dependent rigidification of this more hydrophilic membrane region. The opposite action of aluminum on synaptosomal membrane fluidity, during in vivo and in vitro experiments, appears to be explained by alteration of synaptosomal CH/PL molar ratio, since a significant reduction (approximately 80%) of this parameter occurs during in vivo exposure to aluminum. In conclusion, during in vivo exposure to aluminum, fluidification of hydrophilic regions and reduction of CH/PL molar ratio of presynaptic membranes accompany the accumulation of this cation, which appear to restrict aluminum retention in brain cortex nerve terminals.

Phospholipase D1b and D2a generate structurally identical phosphatidic acid species in mammalian cells

Mammalian cells contain different phospholipase D enzymes (PLDs) whose distinct physiological roles are poorly understood and whose products have not been characterized. The development of porcine aortic endothelial (PAE) cell lines able to overexpress PLD-1b or -2a under the control of an inducible promoter has enabled us to characterize both the substrate specificity and the phosphatidic acid (PtdOH) product of these enzymes under controlled conditions. Liquid chromatography-MS analysis showed that PLD1b- and PLD2a-transfected PAE cells, as well as COS7 and Rat1 cells, generate similar PtdOH and, in the presence of butan-1-ol, phosphatidylbutanol (PtdBut) profiles, enriched in mono- and di-unsaturated species, in particular 16:0/18:1. Although PtdBut mass increased, the species profile did not change in cells stimulated with ATP or PMA. Overexpression of PLD made little difference to basal or stimulated PtdBut formation, indicating that activity is tightly regulated in vivo and that factors other than just PLD protein levels limit hydrolytic function. In vitro assays using PLD-enriched lysates showed that the enzyme could utilize both phosphatidylcholine and, much less efficiently, phosphatidylethanolamine, with slight selectivity towards mono- and di-unsaturated species. Phosphatidylinositol was not a substrate. Thus PLD1b and PLD2a hydrolyse a structurally similar substrate pool to generate an identical PtdOH product enriched in mono- and di-unsaturated species that we propose to function as the intracellular messenger forms of this lipid.

PLD pathway involved in carbachol-induced Cl- secretion: possible role of TNF-alpha

In a previous study, it was found that exposure to tumor necrosis factor-alpha (TNF-alpha) potentiated the electrophysiological response to carbachol in a time-dependent and cycloheximide-sensitive manner. It was deduced that the potentiation could be due to protein kinase C activity because of increased 1,2-diacylglycerol. It was also observed that propranolol could decrease the electrophysiological response to carbachol (Oprins JC, Meijer HP, and Groot JA. Am J Physiol Cell Physiol 278: C463-C472, 2000). The aim of the present study was to investigate whether the phospholipase D (PLD) pathway plays a role in the carbachol response and the potentiating effect of TNF-alpha. The transphosphatidylation reaction in the presence of the primary alcohol 1-butanol [leading to stable phosphatidylbutanol (Pbut) formation] was used to measure activity of PLD. The phosphatidic acid (PA) levels were also measured. Muscarinic stimulation resulted in an increased formation of Pbut and PA. TNF-alpha decreased levels of PA.

Protein kinase C activation by 12-0-tetradecanoylphorbol 13-acetate in CG-4 line oligodendrocytes stimulates turnover of choline and ethanolamine phospholipids by phospholipase D and induces rapid process contraction

Treatment of [3H]-choline- or [14C]-ethanolamine-labelled undifferentiated bipolar and differentiated multipolar CG-4 line oligodendrocytes with 12-0-tetradecanoylphorbol 13-acetate (TPA) to activate protein kinase C stimulated the release of choline or ethanolamine metabolites to the medium over controls. Ro31-8220, a PKC inhibitor, reduced TPA-stimulated release of choline- and ethanolamine-metabolites to basal levels. TPA treatment of both bipolar and multipolar cells caused rapid contraction of processes leaving rounded up cells: this effect was blocked by Ro31-8220. After 12-15 h exposure to TPA, bipolar undifferentiated CG-4 line cells extended short processes again and the cells became multipolar. Nocodozole, an agent which disrupts microtubules and caused CG-4 line cells to round up, caused increased choline or ethanolamine-metabolite release to the medium over basal levels suggesting that some release during TPA-treatment might occur due to process fragmentation. However, the transphosphatidylation reaction confirmed that phospholipase D was active in these cells. Exposure of bipolar undifferentiated CG-4 line cells to TPA resulted in down-regulatation of PKC-alpha and PKC-beta which could not be detected by Western blotting after a few hours; PKC-epsilon was down-regulated much more slowly but PKCs delta, zeta and iota were not influenced by 48 h exposure of cells to TPA. Formation of phosphatidylethanol in the transphosphatidylation reaction was markedly reduced in TPA down-regulated cells indicating a role for PKCs alpha and beta in phospholipase D activation in CG-4 line oligodendrocytes.

Attenuation of intestinal ischemia/reperfusion injury with sodium nitroprusside: studies on mitochondrial function and lipid changes

Reactive oxygen species have been implicated in cellular injury during ischemia/reperfusion (I/R). Mitochondria are one of the main targets of oxygen free radicals and damage to this organelle leads to cell death. Reports suggest that nitric oxide (NO) may offer protection from damage during I/R. This study has looked at the functional changes and lipid alteration to mitochondria during intestinal I/R and the protection offered by NO. It was observed that I/R of the intestine is associated with functional alterations in the mitochondria as suggested by MTT reduction, respiratory control ratio and mitochondrial swelling. Mitochondrial lipid changes suggestive of activation of phospholipase A(2) and phospholipase D were also seen after (I/R) mediated injury. These changes were prevented by the simultaneous presence of a NO donor in the lumen of the intestine. These studies have suggested that structural and functional alterations of mitochondria are prominent features of I/R injury to the intestine which can be ameliorated by NO.

Localization and possible functions of phospholipase D isozymes

The activation of PLD is believed to play an important role in the regulation of cell function and cell fate by extracellular signal molecules. Multiple PLD activities have been characterized in mammalian cells and, more recently, several PLD genes have been cloned. Current evidence indicates that diverse PLD activities are localized in most, if not all, cellular organelles, where they are likely to subserve different functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

Apoptosis in the monkey small intestinal epithelium: structural and functional alterations in the mitochondria

Our earlier studies have shown apoptosis in the villus tip cells of the monkey small intestinal epithelium. Because mitochondria have been implicated in the apoptotic process, this study looked at the function and lipid composition of mitochondria isolated from apoptotic villus tip cells and compared it with middle and crypt cells. Decreased MTT reduction and respiratory control ratio, increased swelling and altered mitochondrial enzyme activities were seen in the villus tip cell mitochondria when compared to other cells. The lipid composition of the villus tip mitochondria were different from the other mitochondria. A decrease in phosphatidylethanolamine and phosphatidyl-inositol and an increase in phosphatidic acid was seen in these mitochondria. Fatty acid composition analysis showed more unsaturated fatty acids in the free fatty acid and phospholipid fraction in villus tip cell mitochondria as compared to other cells. These studies suggest that in the monkey small intestinal epithelium, apoptotic process is associated with functional and structural alterations in the mitochondria.

Overexpression of protein kinase C-epsilon and its regulatory domains in fibroblasts inhibits phorbol ester-induced phospholipase D activity

In fibroblasts, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates phospholipase D (PLD)-mediated hydrolysis of both phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) by PKC-alpha-mediated nonphosphorylating and phosphorylating mechanisms. Here we have used NIH 3T3 fibroblasts overexpressing holo PKC-epsilon and its regulatory, catalytic, and zinc finger domain fragments to determine if this isozyme also regulates PLD activity. Overexpression of holo PKC-epsilon inhibited the stimulatory effects of PMA (5-100 nM) on both PtdCho and PtdEtn hydrolysis. Overexpression of PKC-epsilon also was found to inhibit platelet-derived growth factor-induced PLD activity. Expression of the catalytic unit of PKC-epsilon had no effect on PMA-induced PLD activity. In contrast, expression of both the regulatory domain fragment and the zinc finger domain of PKC-epsilon resulted in significant inhibition of PMA-stimulated PtdCho and PtdEtn hydrolysis. Interestingly, although PKC-alpha also mediates the stimulatory effect of PMA on the synthesis of PtdCho by a phosphorylation mechanism, overexpression of holo PKC-epsilon or its regulatory domain fragments did not affect PMA-induced PtdCho synthesis. These results indicate that the PKC-epsilon system can act as a negative regulator of PLD activity and that this inhibition is mediated by its regulatory domain.

Increased phospholipase D activity in butyrate-induced differentiation of HT-29 cells

Phospholipids are important constituents of biomembrane components and are supposed to function as enzyme activators or precursors of bioactive substances. Our earlier work has shown an increased esterification of neutral lipids of HT-29 cells during butyrate-induced differentiation (M. Madesh, O. Benard, K.A. Balasubramanian, Butyrate-induced alteration in lipid composition of human colon cell line HT-29, Biochem. Mol. Biol. Int. 38 (1996) 659-664). In this report we show that there is an increase in phospholipase D (PLD) activity during butyrate-induced differentiation of HT-29 cells as indicated by the formation of phosphatidic acid (PA). When the control and butyrate-treated cell homogenates were incubated in vitro with 1 mM Ca2+, the increase in PA formation was higher than in butyrate-treated cells. This PA was formed due to PLD activity that was confirmed by the generation of phosphatidylethanol by in vitro incubation of HT-29 cell homogenates in the presence of ethanol. The formation of PA was associated with a decrease in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). This study has shown an increase in PLD activity associated with the differentiation of HT-29 cells.

Metal-ion stimulation and inhibition of lysophospholipase D which generates bioactive lysophosphatidic acid in rat plasma

We found that lysophospholipase D (LPLD) in rat plasma prefers unsaturated to saturated lysophosphatidylcholines as substrates, generating a biologically active lipid, lysophosphatidic acid, but it does not hydrolyze diacyl-phospholipids. In this study, this LPLD required a metal ion for activity, Co2+ being the most effective, followed in order by Zn2+, Mn2+, and Ni2+. This metal-ion-stimulated LPLD with unique substrate specificity, which has not been described previously, was susceptible to thiol-blocking reagents and serine esterase inhibitors, but not to a histidine-modifying reagent. Consistent with results using thiol-modifying agents, short-chain fatty aldehydes, secondary products of lipid peroxidation, were found to inhibit LPLD. Addition of dibutylhydroxytoluene or butylhydroxyanisole to the plasma increased the activity of this enzyme, probably in a manner independent of its antioxidant activity, since another antioxidant, propyl gallate, was rather inhibitory. These results suggest that rat plasma contains an active LPLD that differs in some properties from other members of the known phospholipase D family detected in animal tissues and body fluids.

Phospholipase D: a novel major player in signal transduction

The role of the mammalian phospholipase D (PLD) in the control of key cellular responses has been recognised for a long time, but only recently have there been the reagents to properly study this very important enzyme in the signalling pathways, linking cell agonists with intracellular targets. With the recent cloning of PLD isoenzymes, their association with low-molecular-weight G proteins, protein kinase C and tyrosine kinases, the availability of antibodies and an understanding of the role of PLD product, phosphatidic acid (PA), in cell physiology, the field is gaining momentum. In this review, we will explore the molecular properties of mammalian PLD and its gene(s), the complexity of this enzyme regulation and the myriad physiological roles for PLD and PA and related metabolic products, with particular emphasis on a role in the activation of NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.

Inhibition by aminosalicylates of phosphatidic acid formation induced by superoxide, calcium or spermine in enterocyte mitochondria

Inflammation is associated with oxidative stress and altered cellular calcium homeostasis. Our earlier studies have shown that, increased phosphatidic acid (PA) formation occurred in enterocyte mitochondria when exposed to superoxide, divalent metal ions or polyamines resulting in altered lipid composition. Since aminosalicylates are the drug of choice for gut inflammation, we have tested the effect of aminosalicylates on PA formation by enterocyte mitochondria. When stimulated by superoxide, Ca2+ or spermine, phosphatidyleth-anolamine (PE) degradation and PA formation occurred in enterocyte mitochondria which can be inhibited by aminosalicylates. The inhibition was 50-60% at 0.5-mM concentration and at 1- or 2-mM final concentration, complete inhibition was observed. Both 5-aminosalicylate (5-ASA) and 4-aminosalicylate (4-ASA) showed similar effects. The stimulation of PA formation by calcium or spermine was not due to increased generation of superoxide by mitochondria which was confirmed by measurement of superoxide production by the mitochondria. These studies suggest that in addition to other cellular effects, aminosalicylates may prevent the enterocyte mitochondrial damage by inhibition of PA formation and PE degradation and alteration of mitochondrial lipid composition.

Cyclosporin A inhibits oxidant and calcium stimulated phospholipase D activity in the rat intestinal mitochondria

Mitochondrial swelling and calcium cycling occurs during oxidative stress and can be prevented by cyclosporin A (CysA). Our earlier work has shown that enterocyte mitochondria contains a phospholipase D (PLD) which can be activated by superoxide or calcium. In this study, we have shown that enterocyte mitochondrial PLD activated by these agents can be inhibited by cyclosporin A. This was clearly shown by the absence of phosphatidic acid (PA) formation and phosphatidylethanolamine (PE) degradation. Since this PLD specifically utilizes PE as substrate, PLD activity was also assessed by ethanolamine formation which was inhibited by CysA. CysA also inhibited the cabbage PLD activity as judged by phosphatidylethanol formation. These results suggest that cyclosporin A is an inhibitor of PLD and this may be one of the mechanism by which CysA protects enterocyte mitochondria from oxidative stress.