Activation of intestinal mitochondrial phospholipase D by polyamines and monoamines

Extracellular Spermine Triggers a Rapid Intracellular Phosphatidic Acid Response in Arabidopsis, Involving PLDδ Activation and Stimulating Ion Flux

Polyamines, such as putrescine (Put), spermidine (Spd), and spermine (Spm), are low-molecular-weight polycationic molecules found in all living organisms. Despite the fact that they have been implicated in various important developmental and adaptative processes, their mode of action is still largely unclear. Here, we report that Put, Spd, and Spm trigger a rapid increase in the signaling lipid, phosphatidic acid (PA) in Arabidopsis seedlings but also mature leaves. Using time-course and dose-response experiments, Spm was found to be the most effective; promoting PA responses at physiological (low μM) concentrations. In seedlings, the increase of PA occurred mainly in the root and partly involved the plasma membrane polyamine-uptake transporter (PUT), RMV1. Using a differential ³²P_i-labeling strategy combined with transphosphatidylation assays and T-DNA insertion mutants, we found that phospholipase D (PLD), and in particular PLDδ was the main contributor of the increase in PA. Measuring non-invasive ion fluxes (MIFE) across the root plasma membrane of wild type and pldδ-mutant seedlings, revealed that the formation of PA is linked to a gradual- and transient efflux of K⁺. Potential mechanisms of how PLDδ and the increase of PA are involved in polyamine function is discussed.

Effects of polyamines on mitochondrial Ca2+ transport

Mammalian mitochondria are able to enhance Ca(2+) accumulation in the presence of polyamines by activating the saturable systems of Ca(2+) inward transport and buffering extramitochondrial Ca(2+) concentrations to levels similar to those in the cytosol of resting cells. This effect renders them responsive to regulate free Ca(2+) concentrations in the physioloical range. The mechanism involved is due to a rise in the affinity of the Ca(2+) transport system, induced by polyamines, most probably exhibiting allosteric behaviour. The regulatory site of this mechanism is the so-called S(1) binding site of polyamines, which operates in physiological conditions and is located in the energy well between the two peaks present in the energy profile of mitochondrial spermine transport. Spermine is bidirectionally transported across teh inner membrane by cycling, in which influx and efflux are driven by electrical and pH gradients, respectively. Most probably, polyamine affects the Ca(2+) transport system when it acts from the outside-that is, in the direction of its uniporter channel, in order to reach the S(1) site. Important physiological functions are related to activation of Ca(2+) transport systems by polyamines and their interactions with the S(1) site. These functions include a rise in the metabolic rate for energy supply and modulation of mitochondrial permeability transition induction, with consequent effects on the triggering of the apoptotic pathway.

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.

Activation of N-acylethanolamine-releasing phospholipase D by polyamines

N-acylethanolamines including anandamide (an endogenous ligand of cannabinoid receptors) are biosynthesized from N-acyl-phosphatidylethanolamine (PE) by a phosphodiesterase of the phospholipase D type. The enzyme partially purified from the particulate fraction of rat heart hydrolyzed N-palmitoyl-PE to N-palmitoylethanolamine with a specific activity of 50 nmol/min per mg protein at 37 degrees C in the presence of 10 mM CaCl2. We found that the enzyme was highly activated in dose-dependent manner by polyamines like spermine, spermidine, and putrescine. Spermine was the most potent with an EC50 value around 0.1 mM, and increased the specific enzyme activity 27 fold up to 53 nmol/min per mg protein. However, a synergistic effect of spermine and the known activator (Ca2+ or Triton X-100) was not observed. The spermine-stimulated enzyme was also active with N-arachidonoyl-PE (a precursor of anandamide). Thus, polyamines may function as endogenous activators to control the biosynthesis of anandamide and other N-acylethanolamines.

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.

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.

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.

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.

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.