Structural requirements of lysophospholipid-regulated mitochondrial Ca2+ transport

Cell-Death Metabolites from Cocconeis scutellum var. parva Identified by Integrating Bioactivity-Based Fractionation and Non-Targeted Metabolomic Approaches

Epiphytic diatoms growing in Mediterranean seagrass meadows, particularly those of the genus Cocconeis, are abundant and ecologically significant, even in naturally acidified environments. One intriguing aspect of some benthic diatoms is their production of an unidentified cell-death-promoting compound, which induces destruction of the androgenic gland in Hippolyte inermis Leach, 1816, a shrimp exhibiting protandric hermaphroditism, principally under normal environmental pH levels. The consumption of Cocconeis spp. by this shrimp is vital for maintaining the stability of its natural populations. Although many attempts have been made to reveal the identity of the apoptotic compound, it is still unknown. In this study, we strategically integrated a bioactivity-based fractionation, a metabolomic approach, and two different experimental avenues to identify potential apoptotic metabolites from Cocconeis scutellum var. parva responsible for the sex reversal in H. inermis. Our integrated analysis uncovered two potential candidate metabolites, one putatively identified as a lysophosphatidylglycerol (LPG) (16:1) and the other classified as a fatty acid ester. This is the first time LPG (16:1) has been reported in C. scutellum var. parva and associated with cell-death processes. These candidate metabolites mark substantial progress in elucidating the factors responsible for triggering the removal of the androgenic gland in the early post-larval phases of H. inermis.

Putative roles of Ca(2+) -independent phospholipase A2 in respiratory chain-associated ROS production in brain mitochondria: influence of docosahexaenoic acid and bromoenol lactone

Ca(2+) -independent phospholipase A2 (iPLA2 ) is hypothesized to control mitochondrial reactive oxygen species (ROS) generation. Here, we modulated the influence of iPLA2 -induced liberation of non-esterified free fatty acids on ROS generation associated with the electron transport chain. We demonstrate enzymatic activity of membrane-associated iPLA2 in native, energized rat brain mitochondria (RBM). Theoretically, enhanced liberation of free fatty acids by iPLA2 modulates mitochondrial ROS generation, either attenuating the reversed electron transport (RET) or deregulating the forward electron transport of electron transport chain. For mimicking such conditions, we probed the effect of docosahexaenoic acid (DHA), a major iPLA2 product on ROS generation. We demonstrate that the adenine nucleotide translocase partly mediates DHA-induced uncoupling, and that low micromolar DHA concentrations diminish RET-dependent ROS generation. Uncoupling proteins have no effect, but the adenine nucleotide translocase inhibitor carboxyatractyloside attenuates DHA-linked uncoupling effect on RET-dependent ROS generation. Under physiological conditions of forward electron transport, low micromolar DHA stimulates ROS generation. Finally, exposure of RBM to the iPLA2 inhibitor bromoenol lactone (BEL) enhanced ROS generation. BEL diminished RBM glutathione content. BEL-treated RBM exhibits reduced Ca(2+) retention capacity and partial depolarization. Thus, we rebut the view that iPLA2 attenuates oxidative stress in brain mitochondria. However, the iPLA2 inhibitor BEL has detrimental activities on energy-dependent mitochondrial functions. The Ca(2+) -independent phospholipase A2 (iPLA2 ), a FFA (free fatty acids)-generating membrane-attached mitochondrial phospholipase, is potential to regulate ROS (reactive oxygen species) generation by mitochondria. FFA can either decrease reversed electron transport (RET)-linked or enhance forward electron transport (FET)-linked ROS generation. In the physiological mode of FET, iPLA2 activity increases ROS generation. The iPLA2 inhibitor BEL exerts detrimental effects on energy-dependent mitochondrial functions.

Origins of metabolic complications in obesity: ectopic fat accumulation. The importance of the qualitative aspect of lipotoxicity

PURPOSE OF REVIEW

This study highlights two aspects of the concept of lipotoxicity. First, the metabolic consequences following ectopic fat accumulation are not only determined by the amount of lipid accumulated, but also the quality of lipid species. Second, the existence of allostatic mechanisms operating at cellular and tissue levels, which counterbalance the negative effects of lipid overload.

RECENT FINDINGS

The development of lipidomics has allowed the isolation and identification of a wide range of lipid species. Some are highly reactive and capable of inducing undesirable toxic effects. Here we focus on recent information related to pathways involved in the production of these reactive lipid species, their sites of generation and tropism for specific organelles and the molecular mechanisms through which they exert toxic effects. We describe how cell membranes and the lipid species forming their bilayer constitute the main platform from which reactive lipid species are generated. We propose that strategies aimed at maintaining membrane lipid homeostasis are fundamental to preventing the initiation of metabolically relevant lipotoxicity.

SUMMARY

It is essential to understand the qualitative component of lipid species involved in cellular toxicity and the molecular mechanisms mediating these toxic effects to identify new therapeutic targets.

Redox modulation of Ca2+ signaling in human endothelial and smooth muscle cells in pre-eclampsia

Pre-eclampsia (PE) is a leading cause of maternal hypertension in pregnancy and is associated with fetal growth restriction, premature birth, and fetal and maternal mortality. Activation and dysfunction of the maternal and fetal endothelium in PE appears to be a consequence of increased oxidative stress, resulting from elevated levels of circulating lipid peroxides. Accumulating evidence implicates reactive oxygen species (ROS) in the pathogenesis of vascular dysfunction in PE, perhaps involving a disturbance in intracellular Ca(2+) signaling. Several ion-transport pathways are highly sensitive to oxidative stress, and the resulting modulation of ion transport by ROS will affect intracellular Ca(2+) homeostasis. We review the evidence that changes in ion transport induced by ROS may be linked with abnormalities in Ca(2+)-mediated signal transduction, leading to endothelial and smooth muscle dysfunction in maternal and fetal circulations in PE. As dysregulation of Ca(2+) signaling in fetal umbilical endothelial cells is maintained in culture and embryonic, fetal, and postnatal development is affected by the cellular redox state, we hypothesize that impaired redox signaling in PE may influence "programming" of the fetal cardiovascular system and endothelial function in adulthood.

ASPECTS OF THE CYTOLOGICAL ACTIVITY OF EDELFOSINE, MILTEFOSINE, AND ILMOFOSINE IN LEISHMANIA DONOVANI

To discover the mode of action of alkyl-lysophospholipids in Leishmania donovani, we studied the effects of edelfosine, miltefosine, and ilmofosine on intracellular pH, the parasite's cell cycle, and the induction of apoptosis. The effect of the alkyl-lysophospholipids was combined with that of inhibitors of some pumps and exchange regulators of intracellular pH (Na+/ H+; Cl-/CO- 3; and the Na+/K+ ATPase). The effect of the 3 alkyl-lysophospholipids on intracellular pH was indirect; the primary action occurred in the parasite's cell membrane. To determine intracellular pH, we used flow cytometry for the macrophages and axenic amastigotes and spectrofluorometry for the promastigote forms. Apoptosis and the cell cycle were studied by flow cytometry. Treatment of the extracellular promastigote form of L. donovani with the 3 alkyl-lysophospholipids induced death by apoptosis, whereas in the infected cell they caused necrosis rather than apoptosis. Miltefosine and ilmofosine at doses of 38 microM caused G2/M cell cycle inhibition in L. donovani promastigotes.

Rat Brain Cortex Mitochondria Release Group II Secretory Phospholipase A2 under Reduced Membrane Potential

Activation of brain mitochondrial phospholipase(s) A(2) (PLA(2)) might contribute to cell damage and be involved in neurodegeneration. Despite the potential importance of the phenomenon, the number, identities, and properties of these enzymes are still unknown. Here, we demonstrate that isolated mitochondria from rat brain cortex, incubated in the absence of respiratory substrates, release a Ca(2+)-dependent PLA(2) having biochemical properties characteristic to secreted PLA(2) (sPLA(2)) and immunoreacting with the antibody raised against recombinant type IIA sPLA(2) (sPLA(2)-IIA). Under identical conditions, no release of fumarase in the extramitochondrial medium was observed. The release of sPLA(2) from mitochondria decreases when mitochondria are incubated in the presence of respiratory substrates such as ADP, malate, and pyruvate, which causes an increase of transmembrane potential determined by cytofluorimetric analysis using DiOC(6)(3) as a probe. The treatment of mitochondria with the uncoupler carbonyl cyanide 3-chlorophenylhydrazone slightly enhances sPLA(2) release. The increase of sPLA(2) specific activity after removal of mitochondrial outer membrane indicates that the enzyme is associated with mitoplasts. The mitochondrial localization of the enzyme has been confirmed by electron microscopy in U-251 astrocytoma cells and by confocal laser microscopy in the same cells and in PC-12 cells, where the structurally similar isoform type V-sPLA(2) has mainly nuclear localization. In addition to sPLA(2), mitochondria contain another phospholipase A(2) that is Ca(2+)-independent and sensitive to bromoenol lactone, associated with the outer mitochondrial membrane. We hypothesize that, under reduced respiratory rate, brain mitochondria release sPLA(2)-IIA that might contribute to cell damage.

Relation between accumulation of phospholipase A2 reaction products and Ca2+ release in isolated liver mitochondria

A Ca(2+)-dependent stimulation of mitochondrial phospholipase A2 is often assumed to play a role in mitochondrial Ca2+ release. We sought to clarify this relation by measuring Ca2+ transport and determining phospholipase A2 reaction products from the same sample of isolated, incubated rat liver mitochondria. When mitochondria had accumulated and spontaneously released again Ca2+, most probably by membrane permeability transition, there was no increase of phospholipase A2 reaction products. However, when the incubation was continued after Ca2+ release, significant increases of the content of lysophosphatidylcholine and unesterified fatty acids could be seen. Quinacrine, an inhibitor of phospholipase A2 activity, prevented Ca2+ release and p-hydroxymercuribenzoic acid, an inhibitor of lysophospholipid reesterification, induced a fast release of Ca2+ from isolated mitochondria. Such effects are usually taken as indirect evidence for a participation of phospholipase A2 in mitochondrial Ca2+ release, but analysis of the mitochondrial lipids revealed that no significant changes of the mass of phospholipase A2 reaction products had occurred. These experiments suggest that the accumulation of phospholipase A2 reaction products in mitochondria is the consequence rather than the cause of the membrane permeability transition. Exogenous phospholipase A2 products, lysophosphatidylcholine and arachidonic acid, induced mitochondrial Ca2+ release after a time lag, which decreased with aging of the mitochondrial preparation. The amount of lysophosphatidylcholine taken up by the mitochondria from the incubation medium during these experiments was measured and compared to the amount of lysophosphatidylcholine produced endogenously by mitochondrial phospholipase A2. From these data it appears likely that the amount of lysophosphatidylcholine generated in the mitochondria after the permeability transition is sufficient to sustain the permeable state. An accumulation of mitochondrially generated phospholipase A2 reaction products after the permeability transition could thus be a decisive factor for the limited reversibility of the membrane permeability transition.

Increased lipolytic activity of sera from pre-eclamptic women due to the presence of a lysophospholipase

Sera from pre-eclamptic women exhibit an increased lipolytic activity compared to sera of women with normal pregnancies. The null hypothesis of this study was that the increased release of free fatty acids (FFA) was due to hydrolysis of circulating triglycerides. The nature of the increased lipolytic activity was investigated by incubating sera from pre-eclamptic (PE) and normal pregnant women (C) with various lipid substrates radiolabeled in the FFA position. The release of FFA in PE-sera was not due to hydrolysis of triglycerides or diglycerides. Lysophosphatidylcholine, however, served as substrate for the enhanced lipolytic activity. By using lysophosphatidylcholine with radiolabeled FFA in the sn-1-position we found that 32 +/- 10 nmol FFA ml-1 h-1 was released in PE-sera, compared to 10 +/- 4 nmol FFA ml-1 h-1 in C-sera. This lysophospholipase activity appears independent of Ca2+ and other divalent cations. The increased release of FFA in sera of pre-eclamptic women can be explained by the presence of a lysophospholipase which releases the remaining fatty acid of lysophosphatidylcholine.

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Enhanced enzymatic degradation of radical damaged mitochondrial membrane components

The location of a protein (soluble or membrane-bound) influences the extent of oxidative damage caused by free radicals. It has been established that after radical attack, soluble proteins can become more susceptible to hydrolysis by individual proteinases than native proteins. We have now examined the hydrolytic susceptibility following radical attack of a protein that is located within a membrane environment, mitochondrial monoamine oxidase (MAO). After exposure to oxygen radicals generated by gamma irradiation, hydrolysis of sub-mitochondrial particles (SMP) containing MAO was increased in three respects. First, the generation of small fragments of MAO by the proteinases elastase and trypsin, was enhanced. Second, the generation by these enzymes and by phospholipase A2 of non-sedimentable membrane fragments containing MAO was also increased. Third, autolysis of SMP was enhanced. Hence, proteins located within membranes may become more susceptible to enzymatic degradation following oxidative damage.

Excitable membranes, lipid messengers, and immediate-early genes. Alteration of signal transduction in neuromodulation and neurotrauma

The physical nature of neuronal cells, particularly in the functional and morphological segregation of synapse, soma, and dendrites, imparts special importance on the integrity of their cell membranes for the localization of function, generation of intrinsic second messengers, and plasticity required for adaptation and repair. The component phospholipids of neural membranes are important sources of bioactive mediators that participate in such diverse phenomena as memory formation and cellular damage following trauma. A common role for PAF in these processes is established through the suppressive effects of its antagonists. Furthermore, being both an extracellular and intracellular agonist of phospholipase activation, in addition to being a product of phospholipase activity, PAF assumes a centralized role in the cellular metabolism following neural stimulation. The linkage of PAF to neural immediate-early gene expression, both in vitro and in vivo, suggests that its effects are initiating to long-term formative and reparative processes. Such a common link between destructive and plastic responses provides an important view of cellular and tissue maintenance in the nervous system.