Decreased iPLA2gamma expression induces lipid peroxidation and cell death and sensitizes cells to oxidant-induced apoptosis

Knockdown of iPLA2γ enhances cisplatin-induced apoptosis by increasing ROS-dependent peroxidation of mitochondrial phospholipids in bladder cancer cells

Cisplatin (CDDP) is a platinum-based drug with anti-cancer activity and is widely used as a standard therapy for bladder cancer. It is well known that CDDP causes cell death by increasing the generation of reactive oxygen species (ROS) and lipid peroxidation, but the mechanism of its anti-cancer effects has not been fully elucidated. There are still some problems such as chemoresistance in CDDP therapy. In the present study, we found the expression of Ca2+-independent phospholipase A2γ (iPLA2γ), which has been reported to regulate cellular redox homeostasis by inhibiting lipid peroxide accumulation, in human bladder cancer tissues. Thus, we investigated the effect of iPLA2γ knockdown on CDDP-induced bladder cancer cell death. As a result, we found that iPLA2γ knockdown significantly enhanced CDDP-induced apoptosis, intracellular and mitochondrial ROS production, cytochrome c release and caspase activation in bladder cancer cells. Moreover, mitochondrial membrane potential was decreased and peroxidation of mitochondrial phospholipids was increased by iPLA2γ knockdown. It was also shown that co-treatment of bromoenol lactone, an iPLA2 inhibitor, increased CDDP-induced apoptosis. These results indicated that iPLA2γ plays an important role in protecting bladder cancer cells from CDDP-induced apoptosis, and that iPLA2γ inhibitors might represent a novel strategy in CDDP-based multi-drug therapy.

Novel Insights on Ferroptosis Modulation as Potential Strategy for Cancer Treatment: When Nature Kills

Significance: The multifactorial nature of the mechanisms implicated in cancer development still represents a major issue for the success of established antitumor therapies. The discovery of ferroptosis, a novel form of programmed cell death distinct from apoptosis, along with the identification of the molecular pathways activated during its execution, has led to the uncovering of novel molecules characterized by ferroptosis-inducing properties. Recent advances: As of today, the ferroptosis-inducing properties of compounds derived from natural sources have been investigated and interesting findings have been reported both in vitro and in vivo. Critical Issues: Despite the efforts made so far, only a limited number of synthetic compounds have been identified as ferroptosis inducers, and their utilization is still limited to basic research. In this review, we analyzed the most important biochemical pathways involved in ferroptosis execution, with particular attention to the newest literature findings on canonical and non-canonical hallmarks, together with mechanisms of action of natural compounds identified as novel ferroptosis inducers. Compounds have been classified based on their chemical structure, and modulation of ferroptosis-related biochemical pathways has been reported. Future Directions: The outcomes herein collected represent a fascinating starting point from which to take hints for future drug discovery studies aimed at identifying ferroptosis-inducing natural compounds for anticancer therapies. Antioxid. Redox Signal. 40, 40-85.

Host phospholipid peroxidation fuels ExoU-dependent cell necrosis and supports Pseudomonas aeruginosa-driven pathology

Regulated cell necrosis supports immune and anti-infectious strategies of the body; however, dysregulation of these processes drives pathological organ damage. Pseudomonas aeruginosa expresses a phospholipase, ExoU that triggers pathological host cell necrosis through a poorly characterized pathway. Here, we investigated the molecular and cellular mechanisms of ExoU-mediated necrosis. We show that cellular peroxidised phospholipids enhance ExoU phospholipase activity, which drives necrosis of immune and non-immune cells. Conversely, both the endogenous lipid peroxidation regulator GPX4 and the pharmacological inhibition of lipid peroxidation delay ExoU-dependent cell necrosis and improve bacterial elimination in vitro and in vivo. Our findings also pertain to the ExoU-related phospholipase from the bacterial pathogen Burkholderia thailandensis, suggesting that exploitation of peroxidised phospholipids might be a conserved virulence mechanism among various microbial phospholipases. Overall, our results identify an original lipid peroxidation-based virulence mechanism as a strong contributor of microbial phospholipase-driven pathology.

Identification of lipidomic profiles associated with drug-resistant prostate cancer cells

Background

The association of circulating lipids with clinical outcomes of drug-resistant castration-resistant prostate cancer (DR-CRPC) is not fully understood. While it is known that increases in select lipids correlate to decreased survival, neither the mechanisms mediating these alterations nor the correlation of resistance to drug treatments is well characterized.

Methods

This gap-in-knowledge was addressed using in vitro models of non-cancerous, hormone-sensitive, CRPC and drug-resistant cell lines combined with quantitative LC-ESI-Orbitrap-MS (LC-ESI-MS/MS) lipidomic analysis and subsequent analysis such as Metaboanalyst and Lipid Pathway Enrichment Analysis (LIPEA).

Results

Several lipid regulatory pathways were identified that are associated with Docetaxel resistance in prostate cancer (PCa). These included those controlling glycerophospholipid metabolism, sphingolipid signaling and ferroptosis. In total, 7460 features were identified as being dysregulated between the cell lines studied, and 21 lipid species were significantly altered in drug-resistant cell lines as compared to nonresistant cell lines. Docetaxel resistance cells (PC3-Rx and DU145-DR) had higher levels of phosphatidylcholine (PC), oxidized lipid species, phosphatidylethanolamine (PE), and sphingomyelin (SM) as compared to parent control cells (PC-3 and DU-145). Alterations were also identified in the levels of phosphatidic acid (PA) and diacylglyceride (DAG), whose levels are regulated by Lipin (LPIN), a phosphatidic acid phosphatase that converts PA to DAG. Data derived from cBioPortal demonstrated a population of PCa patients expressing mutations aligning with amplification of LPIN1, LPIN2 and LPIN3 genes. Lipin amplification in these genes correlated to decreased survival in these patients. Lipin-1 mRNA expression also showed a similar trend in PCa patient data. Lipin-1, but not Lipin-2 or − 3, was detected in several prostate cancer cells, and was increased in 22RV1 and PC-3 cell lines. The increased expression of Lipin-1 in these cells correlated with the level of PA.

Conclusion

These data identify lipids whose levels may correlate to Docetaxel sensitivity and progression of PCa. The data also suggest a correlation between the expression of Lipin-1 in cells and patients with regards to prostate cancer cell aggressiveness and patient survivability. Ultimately, these data may be useful for identifying markers of lethal and/or metastatic prostate cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01437-5.

Calcium-independent phospholipase A2γ (iPLA2γ) and its roles in cellular functions and diseases

Calcium-independent phospholipase A₂γ (iPLA₂γ)/patatin-like phospholipase domain-containing lipase 8 (PNPLA8) is one of the iPLA₂ enzymes, which do not require Ca²⁺ ion for their activity. iPLA₂γ is a membrane-bound enzyme with unique features, including the utilization of four distinct translation initiation sites and the presence of mitochondrial and peroxisomal localization signals. This enzyme is preferentially distributed in the mitochondria and peroxisomes and is thought to be responsible for the maintenance of lipid homeostasis in these organelles. Thus, both the overexpression and the deletion of iPLA₂γ in vivo caused mitochondrial abnormalities and dysfunction. Roles of iPLA₂γ in lipid mediator production and cytoprotection against oxidative stress have also been suggested by in vitro and in vivo studies. The dysregulation of iPLA₂γ can therefore be a critical factor in the development of many diseases, including metabolic diseases and cancer. In this review, we provide an overview of the biochemical properties of iPLA₂γ and then summarize the current understanding of the in vivo roles of iPLA₂γ revealed by knockout mouse studies.

Effects of high-fat diet and age on the blood lipidome and circulating endocannabinoids of female C57BL/6 mice

Alterations in lipid metabolism play a significant role in the pathogenesis of obesity-associated disorders, and dysregulation of the lipidome across multiple diseases has prompted research to identify novel lipids indicative of disease progression. To address the significant gap in knowledge regarding the effect of age and diet on the blood lipidome, we used shotgun lipidomics with electrospray ionization-mass spectrometry (ESI-MS). We analyzed blood lipid profiles of female C57BL/6 mice following high-fat diet (HFD) and low-fat diet (LFD) consumption for short (6weeks), long (22weeks), and prolonged (36weeks) periods. We examined endocannabinoid levels, plasma esterase activity, liver homeostasis, and indices of glucose tolerance and insulin sensitivity to compare lipid alterations with metabolic dysregulation. Multivariate analysis indicated differences in dietary blood lipid profiles with the most notable differences after 6weeks along with robust alterations due to age. HFD altered phospholipids, fatty acyls, and glycerolipids. Endocannabinoid levels were affected in an age-dependent manner, while HFD increased plasma esterase activity at all time points, with the most pronounced effect at 6weeks. HFD-consumption also altered liver mRNA levels of PPARα, PPARγ, and CD36. These findings indicate an interaction between dietary fat consumption and aging with widespread effects on the lipidome, which may provide a basis for identification of female-specific obesity- and age-related lipid biomarkers.

Physical exercise, reactive oxygen species and neuroprotection

Regular exercise has systemic beneficial effects, including the promotion of brain function. The adaptive response to regular exercise involves the up-regulation of the enzymatic antioxidant system and modulation of oxidative damage. Reactive oxygen species (ROS) are important regulators of cell signaling. Exercise, via intensity-dependent modulation of metabolism and/or directly activated ROS generating enzymes, regulates the cellular redox state of the brain. ROS are also involved in the self-renewal and differentiation of neuronal stem cells and the exercise-mediated neurogenesis could be partly associated with ROS production. Exercise has strong effects on the immune system and readily alters the production of cytokines. Certain cytokines, especially IL-6, IL-1, TNF-α, IL-18 and IFN gamma, are actively involved in the modulation of synaptic plasticity and neurogenesis. Cytokines can also contribute to ROS production. ROS-mediated alteration of lipids, protein, and DNA could directly affect brain function, while exercise modulates the accumulation of oxidative damage. Oxidative alteration of macromolecules can activate signaling processes, membrane remodeling, and gene transcription. The well known neuroprotective effects of exercise are partly due to redox-associated adaptation.

Genetic Ablation of Calcium-independent Phospholipase A2γ Induces Glomerular Injury in Mice

Glomerular visceral epithelial cells (podocytes) play a critical role in the maintenance of glomerular permselectivity. Podocyte injury, manifesting as proteinuria, is the cause of many glomerular diseases. We reported previously that calcium-independent phospholipase A2γ (iPLA2γ) is cytoprotective against complement-mediated glomerular epithelial cell injury. Studies in iPLA2γ KO mice have demonstrated an important role for iPLA2γ in mitochondrial lipid turnover, membrane structure, and metabolism. The aim of the present study was to employ iPLA2γ KO mice to better understand the role of iPLA2γ in normal glomerular and podocyte function as well as in glomerular injury. We show that deletion of iPLA2γ did not cause detectable albuminuria; however, it resulted in mitochondrial structural abnormalities and enhanced autophagy in podocytes as well as loss of podocytes in aging KO mice. Moreover, after induction of anti-glomerular basement membrane nephritis in young mice, iPLA2γ KO mice exhibited significantly increased levels of albuminuria, podocyte injury, and loss of podocytes compared with wild type. Thus, iPLA2γ has a protective functional role in the normal glomerulus and in glomerulonephritis. Understanding the role of iPLA2γ in glomerular pathophysiology provides opportunities for the development of novel therapeutic approaches to glomerular injury and proteinuria.

Extraction, chromatographic and mass spectrometric methods for lipid analysis

Lipids make up a diverse subset of biomolecules that are responsible for mediating a variety of structural and functional properties as well as modulating cellular functions such as trafficking, regulation of membrane proteins and subcellular compartmentalization. In particular, phospholipids are the main constituents of biological membranes and play major roles in cellular processes like transmembrane signaling and structural dynamics. The chemical and structural variety of lipids makes analysis using a single experimental approach quite challenging. Research in the field relies on the use of multiple techniques to detect and quantify components of cellular lipidomes as well as determine structural features and cellular organization. Understanding these features can allow researchers to elucidate the biochemical mechanisms by which lipid-lipid and/or lipid-protein interactions take place within the conditions of study. Herein, we provide an overview of essential methods for the examination of lipids, including extraction methods, chromatographic techniques and approaches for mass spectrometric analysis.

Confirmation of a non-synonymous SNP in PNPLA8 as a candidate causal mutation for Weaver syndrome in Brown Swiss cattle

Background

Bovine progressive degenerative myeloencephalopathy (Weaver syndrome) is a neurodegenerative disorder in Brown Swiss cattle that is characterized by progressive hind leg weakness and ataxia, while sensorium and spinal reflexes remain unaffected. Although the causal mutation has not been identified yet, an indirect genetic test based on six microsatellite markers and consequent exclusion of Weaver carriers from breeding have led to the complete absence of new cases for over two decades. Evaluation of disease status by imputation of 41 diagnostic single nucleotide polymorphisms (SNPs) and a common haplotype published in 2013 identified several suspected carriers in the current breeding population, which suggests a higher frequency of the Weaver allele than anticipated. In order to prevent the reemergence of the disease, this study aimed at mapping the gene that underlies Weaver syndrome and thus at providing the basis for direct genetic testing and monitoring of today’s Braunvieh/Brown Swiss herds.

Results

Combined linkage/linkage disequilibrium mapping on Bos taurus chromosome (BTA) 4 based on Illumina Bovine SNP50 genotypes of 43 Weaver-affected, 31 Weaver carrier and 86 Weaver-free animals resulted in a maximum likelihood ratio test statistic value at position 49,812,384 bp. The confidence interval (0.853 Mb) determined by the 2-LOD drop-off method was contained within a 1.72-Mb segment of extended homozygosity. Exploitation of whole-genome sequence data from two official Weaver carriers and 1145 other bulls that were sequenced in Run4 of the 1000 bull genomes project showed that only a non-synonymous SNP (rs800397662) within the PNPLA8 gene at position 49,878,773 bp was concordant with the Weaver carrier status. Targeted SNP genotyping confirmed this SNP as a candidate causal mutation for Weaver syndrome. Genotyping for the candidate causal mutation in a random sample of 2334 current Braunvieh animals suggested a frequency of the Weaver allele of 0.26 %.

Conclusions

Through combined use of exhaustive sequencing data and SNP genotyping results, we were able to provide evidence that supports the non-synonymous mutation at position 49,878,773 bp as the most likely causal mutation for Weaver syndrome. Further studies are needed to uncover the exact mechanisms that underlie this syndrome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-016-0201-5) contains supplementary material, which is available to authorized users.

Calcium-independent phospholipases A2 in murine osteoblastic cells and their inhibition by bromoenol lactone: impact on arachidonate dynamics and prostaglandin synthesis

CONTEXT

Bromoenol lactone (BEL) is an inhibitor of group VI phospholipases (iPLA2s), but has been shown to have severe side effects.

OBJECTIVE

iPLA2 characterization in osteoblasts and effect of BEL on prostaglandin (PG) E2 formation.

METHODS

iPLA2 expression: RT-PCR, Western Blotting. PGE2 formation: GC-MS after stimulation, treatment with inhibitors or gene silencing. Arachidonate (AA) reacylation into phospholipids, inhibitor reaction products, PGHS-1 modification proteomic analysis: HR-LC-MS/MS. AA accumulation: (14)C-AA.

RESULTS

iPLA2ß and iPLA2γ were expressed and functionally active. BEL inhibition up to 20 μM caused AA accumulation and enhanced PGE2 formation, followed by a decrease at higher concentrations. BEL reacted with intracellular cysteine and GSH leading to GSH depletion and oxidative stress.

DISCUSSION

Initial PGE2 enhancement after BEL inhibition is due to iPLA2-independent accumulation of AA. GSH depletion caused by high BEL concentrations is responsible for the decrease in PGE2 production.

CONCLUSION

BEL must be used with caution in a cellular environment due to conditions of extreme oxidative stress.

Differential effects of cocaine exposure on the abundance of phospholipid species in rat brain and blood

BACKGROUND

Lipid profiles in the blood are altered in human cocaine users, suggesting that cocaine exposure can induce lipid remodeling.

METHODS

Lipid changes in the brain tissues of rats sensitized to cocaine were determined through shotgun lipidomics using electrospray ionization-mass spectrometry (ESI-MS). We also performed pairwise principal component analysis (PCA) to assess cocaine-induced changes in blood lipid profiles. Alterations in the abundance of phospholipid species were correlated with behavioral changes in the magnitude of either the initial response to the drug or locomotor sensitization.

RESULTS

Behavioral sensitization altered the relative abundance of several phospholipid species in the hippocampus and cerebellum, measured one week following the final exposure to cocaine. In contrast, relatively few effects on phospholipids in either the dorsal or the ventral striatum were observed. PCA analysis demonstrated that cocaine altered the relative abundance of several glycerophospholipid species as compared to saline-injected controls in blood. Subsequent MS/MS analysis identified some of these lipids as phosphatidylethanolamines, phosphatidylserines and phosphatidylcholines. The relative abundance of some of these phospholipid species were well-correlated (R(2) of 0.7 or higher) with either the initial response to cocaine or locomotor sensitization.

CONCLUSION

Taken together, these data demonstrate that a cocaine-induced sensitization assay results in the remodeling of specific phospholipids in rat brain tissue in a region-specific manner and also alters the intensities of certain types of phospholipid species in rat blood. These results further suggest that such changes may serve as biomarkers to assess the neuroadaptations occurring following repeated exposure to cocaine.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Calcium-independent phospholipase A2γ enhances activation of the ATF6 transcription factor during endoplasmic reticulum stress

Injury of visceral glomerular epithelial cells (GECs) causes proteinuria in many glomerular diseases. We reported previously that calcium-independent phospholipase A2γ (iPLA2γ) is cytoprotective against complement-mediated GEC injury. Because iPLA2γ is localized at the endoplasmic reticulum (ER), this study addressed whether the cytoprotective effect of iPLA2γ involves the ER stress unfolded protein response (UPR). In cultured rat GECs, overexpression of the full-length iPLA2γ, but not a mutant iPLA2γ that fails to associate with the ER, augmented tunicamycin-induced activation of activating transcription factor-6 (ATF6) and induction of the ER chaperones, glucose-regulated protein 94 (GRP94) and glucose-regulated protein 78 (GRP78). Augmented responses were inhibited by the iPLA2γ inhibitor, (R)-bromoenol lactone, but not by the cyclooxygenase inhibitor, indomethacin. Tunicamycin-induced cytotoxicity was reduced in GECs expressing iPLA2γ, and the cytoprotection was reversed by dominant-negative ATF6. GECs from iPLA2γ knock-out mice showed blunted ATF6 activation and chaperone up-regulation in response to tunicamycin. Unlike ATF6, the two other UPR pathways, i.e. inositol-requiring enzyme 1α and protein kinase RNA-like ER kinase pathways, were not affected by iPLA2γ. Thus, in GECs, iPLA2γ amplified activation of the ATF6 pathway of the UPR, resulting in up-regulation of ER chaperones and cytoprotection. These effects were dependent on iPLA2γ catalytic activity and association with the ER but not on prostanoids. Modulating iPLA2γ activity may provide opportunities for pharmacological intervention in glomerular diseases associated with ER stress.

Neuroinflammation and neurodegeneration in adult rat brain from binge ethanol exposure: abrogation by docosahexaenoic acid

Evidence that brain edema and aquaporin-4 (AQP4) water channels have roles in experimental binge ethanol-induced neurodegeneration has stimulated interest in swelling/edema-linked neuroinflammatory pathways leading to oxidative stress. We report here that neurotoxic binge ethanol exposure produces comparable significant effects in vivo and in vitro on adult rat brain levels of AQP4 as well as neuroinflammation-linked enzymes: key phospholipase A2 (PLA2) family members and poly (ADP-ribose) polymerase-1 (PARP-1). In adult male rats, repetitive ethanol intoxication (3 gavages/d for 4 d, ∼9 g/kg/d, achieving blood ethanol levels ∼375 mg/dl; “Majchrowicz” model) significantly increased AQP4, Ca⁺²-dependent PLA2 GIVA (cPLA2), phospho-cPLA2 GIVA (p-cPLA2), secretory PLA2 GIIA (sPLA2) and PARP-1 in regions incurring extensive neurodegeneration in this model—hippocampus, entorhinal cortex, and olfactory bulb—but not in two regions typically lacking neurodamage, frontal cortex and cerebellum. Also, ethanol reduced hippocampal Ca⁺²-independent PLA2 GVIA (iPLA2) levels and increased brain “oxidative stress footprints” (4-hydroxynonenal-adducted proteins). For in vitro studies, organotypic cultures of rat hippocampal-entorhinocortical slices of adult age (∼60 d) were ethanol-binged (100 mM or ∼450 mg/dl) for 4 d, which augments AQP4 and causes neurodegeneration (Collins et al. 2013). Reproducing the in vivo results, cPLA2, p-cPLA2, sPLA2 and PARP-1 were significantly elevated while iPLA2 was decreased. Furthermore, supplementation with docosahexaenoic acid (DHA; 22:6n-3), known to quell AQP4 and neurodegeneration in ethanol-treated slices, blocked PARP-1 and PLA2 changes while counteracting endogenous DHA reduction and increases in oxidative stress footprints (3-nitrotyrosinated proteins). Notably, the PARP-1 inhibitor PJ-34 suppressed binge ethanol-dependent neurodegeneration, indicating PARP upstream involvement. The results with corresponding models support involvement of AQP4- and PLA2-associated neuroinflammatory pro-oxidative pathways in the neurodamage, with potential regulation by PARP-1 as well. Furthermore, DHA emerges as an effective inhibitor of these binge ethanol-dependent neuroinflammatory pathways as well as associated neurodegeneration in adult-age brain.

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.

Group VIA phospholipase A2 mitigates palmitate-induced β-cell mitochondrial injury and apoptosis

Palmitate (C16:0) induces apoptosis of insulin-secreting β-cells by processes that involve generation of reactive oxygen species, and chronically elevated blood long chain free fatty acid levels are thought to contribute to β-cell lipotoxicity and the development of diabetes mellitus. Group VIA phospholipase A2 (iPLA2β) affects β-cell sensitivity to apoptosis, and here we examined iPLA2β effects on events that occur in β-cells incubated with C16:0. Such events in INS-1 insulinoma cells were found to include activation of caspase-3, expression of stress response genes (C/EBP homologous protein and activating transcription factor 4), accumulation of ceramide, loss of mitochondrial membrane potential, and apoptosis. All of these responses were blunted in INS-1 cells that overexpress iPLA2β, which has been proposed to facilitate repair of oxidized mitochondrial phospholipids, e.g. cardiolipin (CL), by excising oxidized polyunsaturated fatty acid residues, e.g. linoleate (C18:2), to yield lysophospholipids, e.g. monolysocardiolipin (MLCL), that can be reacylated to regenerate the native phospholipid structures. Here the MLCL content of mouse pancreatic islets was found to rise with increasing iPLA2β expression, and recombinant iPLA2β hydrolyzed CL to MLCL and released oxygenated C18:2 residues from oxidized CL in preference to native C18:2. C16:0 induced accumulation of oxidized CL species and of the oxidized phospholipid (C18:0/hydroxyeicosatetraenoic acid)-glycerophosphoethanolamine, and these effects were blunted in INS-1 cells that overexpress iPLA2β, consistent with iPLA2β-mediated removal of oxidized phospholipids. C16:0 also induced iPLA2β association with INS-1 cell mitochondria, consistent with a role in mitochondrial repair. These findings indicate that iPLA2β confers significant protection of β-cells against C16:0-induced injury.

Oxygen consumption and usage during physical exercise: the balance between oxidative stress and ROS-dependent adaptive signaling

The complexity of human DNA has been affected by aerobic metabolism, including endurance exercise and oxygen toxicity. Aerobic endurance exercise could play an important role in the evolution of Homo sapiens, and oxygen was not important just for survival, but it was crucial to redox-mediated adaptation. The metabolic challenge during physical exercise results in an elevated generation of reactive oxygen species (ROS) that are important modulators of muscle contraction, antioxidant protection, and oxidative damage repair, which at moderate levels generate physiological responses. Several factors of mitochondrial biogenesis, such as peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), mitogen-activated protein kinase, and SIRT1, are modulated by exercise-associated changes in the redox milieu. PGC-1α activation could result in decreased oxidative challenge, either by upregulation of antioxidant enzymes and/or by an increased number of mitochondria that allows lower levels of respiratory activity for the same degree of ATP generation. Endogenous thiol antioxidants glutathione and thioredoxin are modulated with high oxygen consumption and ROS generation during physical exercise, controlling cellular function through redox-sensitive signaling and protein-protein interactions. Endurance exercise-related angiogenesis, up to a significant degree, is regulated by ROS-mediated activation of hypoxia-inducible factor 1α. Moreover, the exercise-associated ROS production could be important to DNA methylation and post-translation modifications of histone residues, which create heritable adaptive conditions based on epigenetic features of chromosomes. Accumulating data indicate that exercise with moderate intensity has systemic and complex health-promoting effects, which undoubtedly involve regulation of redox homeostasis and signaling.

Effect of repetitive daily ethanol intoxication on adult rat brain: significant changes in phospholipase A2 enzyme levels in association with increased PARP-1 indicate neuroinflammatory pathway activation

Collaborating on studies of subchronic daily intoxication in juvenile and adult rats, we examined whether the repetitive ethanol treatments at these two life stages altered levels of key neuroinflammation-associated proteins-aquaporin-4 (AQP4), certain phospholipase A2 (PLA2) enzymes, PARP-1 and caspase-3-in hippocampus (HC) and entorhinal cortex (EC). Significant changes in the proteins could implicate activation of specific neuroinflammatory signaling pathways in these rats as well as in severely binge-intoxicated adult animals that are reported to incur degeneration of vulnerable neurons in HC and EC. Male Wistar rats, ethanol-intoxicated (3 g/kg i.p.) once daily for 6 days over an 8-day interval beginning at 37 days old and repeated at age 68-75 days, were sacrificed 1 h after the day 75 dose (blood ethanol, 200- 230 mg/dl). Analysis of HC with an immunoblot technique showed that AQP4, Ca(+2)-dependent PLA2 (cPLA2 IVA), phosphorylated (activated) p-cPLA2, cleaved (89 kD) PARP (c-PARP), and caspase-3 levels were significantly elevated over controls, whereas Ca(+2)-independent PLA2 (iPLA2 VIA) was reduced ∼70%; however, cleaved caspase-3 was undetectable. In the EC, AQP4 was unchanged, but cPLA2 and p-cPLA2 were significantly increased while iPLA2 levels were diminished (∼40%) similar to HC, although just outside statistical significance (p = 0.06). In addition, EC levels of PARP-1 and c-PARP were significantly increased. The ethanol-induced activation of cPLA2 in association with reduced iPLA2 mirrors PLA2 changes in reports of neurotrauma and also of dietary omega-3 fatty acid depletion. Furthermore, the robust PARP-1 elevations accompanied by negligible caspase-3 activation indicate that repetitive ethanol intoxication may be potentiating non-apoptotic neurodegenerative processes such as parthanatos. Overall, the repetitive ethanol treatments appeared to instigate previously unappreciated neuroinflammatory pathways in vivo. The data provide insights into mechanisms of binge ethanol abuse that might suggest new therapeutic approaches to counter neurodegeneration and dementia.

Antioxidant activity by a synergy of redox-sensitive mitochondrial phospholipase A2 and uncoupling protein-2 in lung and spleen

Mitochondrial uncoupling protein-2 (UCP2) has been suggested to participate in the attenuation of the reactive oxygen species production, but the mechanism of action and the physiological significance of UCP2 activity remain controversial. Here we tested the hypothesis that UCP2 provides feedback downregulation of oxidative stress in vivo via synergy with an H2O2-activated mitochondrial calcium-independent phospholipase A2 (mt-iPLA2). Tert-butylhydroperoxide or H2O2 induced free fatty acid release from mitochondrial membranes as detected by gas chromatography/mass spectrometry, which was inhibited by r-bromoenol lactone (r-BEL) but not by its stereoisomer s-BEL, suggesting participation of mt-iPLA2γ isoform. Tert-butylhydroperoxide or H2O2 also induced increase in respiration and decrease in mitochondrial membrane potential in lung and spleen mitochondria from control but not UCP2-knockout mice. These data suggest that mt-iPLA2γ-dependent release of free fatty acids promotes UCP2-dependent uncoupling. Upon such uncoupling, mitochondrial superoxide formation decreased instantly also in the s-BEL presence, but not when mt-iPLA2 was blocked by R-BEL and not in mitochondria from UCP2-knockout mice. Mt-iPLA2γ was alternatively activated by H2O2 produced probably in conjunction with the electron-transferring flavoprotein:ubiquinone oxidoreductase (ETFQOR), acting in fatty acid β-oxidation. Palmitoyl-d,l-carnitine addition to mouse lung mitochondria, respiring with succinate plus rotenone, caused a respiration increase that was sensitive to r-BEL and insensitive to s-BEL. We thus demonstrate for the first time that UCP2, functional due to fatty acids released by redox-activated mt-iPLA2γ, suppresses mitochondrial superoxide production by its uncoupling action. In conclusion, H2O2-activated mt-iPLA2γ and UCP2 act in concert to protect against oxidative stress.

Complement-mediated activation of calcium-independent phospholipase A2γ: role of protein kinases and phosphorylation

In experimental membranous nephropathy, complement C5b-9-induces glomerular epithelial cell (GEC) injury and proteinuria. The effects of C5b-9 are mediated via signaling pathways, including calcium-independent phospholipase A(2)γ (iPLA(2)γ), and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. The iPLA(2)γ pathway is cytoprotective. This study addresses the mechanisms of iPLA(2)γ activation. iPLA(2)γ activity was monitored by quantifying prostaglandin E(2) (PGE(2)) production. In GECs, iPLA(2)γ localized at the endoplasmic reticulum and mitochondria. Complement-mediated production of PGE(2) was amplified in GECs that overexpress iPLA(2)γ, compared with control cells, and was blocked by the iPLA(2)γ inhibitor bromoenol lactone in both iPLA(2)γ-overexpressing and control GECs. In GECs that overexpress iPLA(2)γ, complement-mediated PGE(2) production was reduced by inhibitors of MAP/ERK kinase 1 (MEK1) and p38 but not JNK. In COS-1 cells that overexpress iPLA(2)γ and cyclooxygenase-1, PGE(2) production was induced by co-expression of constitutively active MEK1 or MAPK-interacting kinase 1 (MNK1) as well as by stimulation with epidermal growth factor (EGF) + ionomycin. Complement- and EGF + ionomycin-stimulated iPLA(2)γ activity was attenuated by the S511A/S515A double mutation. Moreover, complement and EGF + ionomycin enhanced phosphorylation of Ser-511. Thus, complement-mediated activation of iPLA(2)γ is mediated via ERK and p38 pathways, and phosphorylation of Ser-511 and/or Ser-515 plays a key role in the catalytic activity and signaling of iPLA(2)γ. Defining the mechanisms by which complement activates iPLA(2)γ provides opportunities for development of novel therapeutic approaches to GEC injury and proteinuria.

Group VIB Phospholipase A(2) promotes proliferation of INS-1 insulinoma cells and attenuates lipid peroxidation and apoptosis induced by inflammatory cytokines and oxidant agents

Group VIB Phospholipase A(2) (iPLA(2)γ) is distributed in membranous organelles in which β-oxidation occurs, that is, mitochondria and peroxisomes, and is expressed by insulin-secreting pancreatic islet β-cells and INS-1 insulinoma cells, which can be injured by inflammatory cytokines, for example, IL-1β and IFN-γ, and by oxidants, for example, streptozotocin (STZ) or t-butyl-hydroperoxide (TBHP), via processes pertinent to mechanisms of β-cell loss in types 1 and 2 diabetes mellitus. We find that incubating INS-1 cells with IL-1β and IFN-γ, with STZ, or with TBHP causes increased expression of iPLA(2)γ mRNA and protein. We prepared INS-1 knockdown (KD) cell lines with reduced iPLA(2)γ expression, and they proliferate more slowly than control INS-1 cells and undergo increased membrane peroxidation in response to cytokines or oxidants. Accumulation of oxidized phospholipid molecular species in STZ-treated INS-1 cells was demonstrated by LC/MS/MS scanning, and the levels in iPLA(2)γ-KD cells exceeded those in control cells. iPLA(2)γ-KD INS-1 cells also exhibited higher levels of apoptosis than control cells when incubated with STZ or with IL-1β and IFN-γ. These findings suggest that iPLA(2)γ promotes β-cell proliferation and that its expression is increased during inflammation or oxidative stress as a mechanism to mitigate membrane injury that may enhance β-cell survival.

Maintenance of synaptic stability requires calcium-independent phospholipase A₂ activity

Phospholipases A₂ (PLA₂s) represent one of the largest groups of lipid-modifying enzymes. Over the years, significant advances have been made in understanding their potential physiological and pathological functions. Depending on their calcium requirement for activation, PLA₂s are classified into calcium dependent and independent. This paper mainly focuses on brain calcium-independent PLA₂ (iPLA₂) and on the mechanisms by which they influence neuronal function and regulate synaptic plasticity. Particular attention will be given to the iPLA₂γ isoform and its role in the regulation of synaptic glutamate receptors. In particular, the paper discusses the possibility that brain iPLA₂γ deficiencies could destabilise normal synaptic operation and might contribute to the aetiology of some brain disorders. In this line, the paper presents new data indicating that iPLA₂γ deficiencies accentuate AMPA receptor destabilization and tau phosphorylation, which suggests that this iPLA₂ isoform should be considered as a potential target for the treatment of Tau-related disorders.

The role of endoplasmic reticulum Ca2+-independent phospholipase a2γ in oxidant-induced lipid peroxidation, Ca2+ release, and renal cell death

Oxidant-induced lipid peroxidation and cell death are major components of ischemia/reperfusion and toxicant injury. Our previous studies showed that renal proximal tubular cells (RPTCs) express Ca(2+)-independent phospholipase A(2)γ (iPLA(2)γ) in endoplasmic reticulum (ER) and mitochondria and that iPLA(2)γ is cytoprotective. Our present studies reveal the role of ER-iPLA(2)γ in oxidant-induced ER lipid peroxidation, Ca(2+) release, and cell death. Oxidant tert-butyl hydroperoxide (TBHP) caused ER lipid peroxidation and Ca(2+) release in isolated rabbit kidney cortex microsomes. ER-iPLA(2)γ inhibition, using bromoenol lactone (BEL), potentiated both oxidant-induced ER lipid peroxidation and Ca(2+) release. Assessment of fatty acids using electrospray ionization-mass spectrometry revealed that ER-iPLA(2)γ mediates the TBHP-induced release of arachidonic acid (20:4), linoleic acid (18:2), and their oxidized forms (18:2-OH, 18:2-OOH, 20:4-OH, 20:4-OOH, 20:4-(OH)(3). iPLA(2)γ inhibition also accelerated oxidant-induced ER Ca(2+) release in RPTC. Depletion of ER Ca(2+) stores in RPTC with thapsigargin, an ER Ca(2+) pump inhibitor, prior to TBHP exposure reduced necrotic cell death and blocked the potentiation of TBHP-induced necrotic cell death by BEL. Together, these data provide strong evidence that ER-iPLA(2)γ protects renal cells from oxidant-induced necrotic cell death by releasing unsaturated and/or oxidized fatty acids from ER membranes, thereby preserving ER membrane integrity and preventing ER Ca(2+) release.

Group VIA phospholipase A2 is a target for vasopressin signaling in the thick ascending limb

Na+-K+-2Cl− cotransporter (NKCC2)-mediated NaCl reabsorption in the thick ascending limb (TAL) is stimulated by AVP via V2 receptor/PKA/cAMP signaling. This process is antagonized by locally produced eicosanoids such as 20-HETE or prostaglandin E2, which are synthesized in a phospholipase A2-dependent reaction cascade. Using microarray-based gene expression analysis, we found evidence for an AVP-dependent downregulation of the calcium-independent isoform of PLA2, iPLA2β, in the outer medulla of rats. In the present study, we therefore examined the contribution of iPLA2β to NKCC2 regulation. Immunoreactive iPLA2β protein was detected in cultured mTAL cells as well as in the entire TAL of rodents and humans with the exception of the macula densa. Administration of the V2 receptor-selective agonist desmopressin (5 ng/h; 3 days) to AVP-deficient diabetes insipidus rats increased outer medullary phosphorylated NKCC2 (pNKCC2) levels more than twofold in association with a marked reduction in iPLA2β abundance (−65%; P < 0.05), thus confirming microarray results. Inhibition of iPLA2β in Sprague-Dawley rats with FKGK 11 (0.5 μM) or in mTAL cells with FKGK 11 (10 μM) or ( S)-bromoenol lactone (5 μM) for 1 h markedly increased pNKCC2 levels without affecting total NKCC2 expression. Collectively, these data indicate that iPLA2β acts as an inhibitory modulator of NKCC2 activity and suggest that downregulation of iPLA2β may be a relevant step in AVP-mediated urine concentration.

Activation of mitochondrial calcium-independent phospholipase A2γ (iPLA2γ) by divalent cations mediating arachidonate release and production of downstream eicosanoids

Calcium-independent phospholipase A(2)γ (iPLA(2)γ) (PNPLA8) is the predominant phospholipase activity in mammalian mitochondria. However, the chemical mechanisms that regulate its activity are unknown. Here, we utilize iPLA(2)γ gain of function and loss of function genetic models to demonstrate the robust activation of iPLA(2)γ in murine myocardial mitochondria by Ca(2+) or Mg(2+) ions. Calcium ion stimulated the production of 2-arachidonoyl-lysophosphatidylcholine (2-AA-LPC) from 1-palmitoyl-2-[(14)C]arachidonoyl-sn-glycero-3-phosphocholine during incubations with wild-type heart mitochondrial homogenates. Furthermore, incubation of mitochondrial homogenates from transgenic myocardium expressing iPLA(2)γ resulted in 13- and 25-fold increases in the initial rate of radiolabeled 2-AA-LPC and arachidonic acid (AA) production, respectively, in the presence of calcium ion. Mass spectrometric analysis of the products of calcium-activated hydrolysis of endogenous mitochondrial phospholipids in transgenic iPLA(2)γ mitochondria revealed the robust production of AA, 2-AA-LPC, and 2-docosahexaenoyl-LPC that was over 10-fold greater than wild-type mitochondria. The mechanism-based inhibitor (R)-(E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one (BEL) (iPLA(2)γ selective), but not its enantiomer, (S)-BEL (iPLA(2)β selective) or pyrrolidine (cytosolic PLA(2)α selective), markedly attenuated Ca(2+)-dependent fatty acid release and polyunsaturated LPC production. Moreover, Ca(2+)-induced iPLA(2)γ activation was accompanied by the production of downstream eicosanoid metabolites that were nearly completely ablated by (R)-BEL or by genetic ablation of iPLA(2)γ. Intriguingly, Ca(2+)-induced iPLA(2)γ activation was completely inhibited by long-chain acyl-CoA (IC(50) ∼20 μm) as well as by a nonhydrolyzable acyl-CoA thioether analog. Collectively, these results demonstrate that mitochondrial iPLA(2)γ is activated by divalent cations and inhibited by acyl-CoA modulating the generation of biologically active metabolites that regulate mitochondrial bioenergetic and signaling functions.

Profiling of fatty acids released during calcium-induced mitochondrial permeability transition in isolated rabbit kidney cortex mitochondria

Increases in intracellular Ca(2+) during cellular stress often lead to the mitochondrial permeability transition (MPT). We examined changes in fatty acids (FAs) released from isolated renal cortical mitochondria subjected to Ca(2+)-induced MPT. Exposing mitochondria to Ca(2+) stimulated mitochondrial swelling and release of FAs such as arachidonic (20:4) and docosahexenoic acids which increased 71% and 32%, respectively, and linoleic (18:2) which decreased 23% compared to controls. Stearic (18:0), oleic (18:1), and linoleic (18:3) acids were unchanged. To elucidate a mechanism for FA release, mitochondria were pre-treated with bromoenolactone (BEL) to inhibit Ca(2+)-independent phospholipase A(2) gamma activity (iPLA(2)γ). BEL blocked Ca(2+)-induced release of arachidonic and behenic (22:0) acids. Finally, four FAs were released in the absence of Ca(2+) in a BEL-sensitive manner, including arachidonic and docosatrienoic acids. Thus, extensive FA release occurs during Ca(2+)-induced MPT, and that mitochondrial iPLA(2)γ maintains mitochondrial arachidonic acid homeostasis under both basal and Ca(2+)-induced stress conditions.

Regulators of G-Protein signaling RGS10 and RGS17 regulate chemoresistance in ovarian cancer cells

BACKGROUND

A critical therapeutic challenge in epithelial ovarian carcinoma is the development of chemoresistance among tumor cells following exposure to first line chemotherapeutics. The molecular and genetic changes that drive the development of chemoresistance are unknown, and this lack of mechanistic insight is a major obstacle in preventing and predicting the occurrence of refractory disease. We have recently shown that Regulators of G-protein Signaling (RGS) proteins negatively regulate signaling by lysophosphatidic acid (LPA), a growth factor elevated in malignant ascites fluid that triggers oncogenic growth and survival signaling in ovarian cancer cells. The goal of this study was to determine the role of RGS protein expression in ovarian cancer chemoresistance.

RESULTS

In this study, we find that RGS2, RGS5, RGS10 and RGS17 transcripts are expressed at significantly lower levels in cells resistant to chemotherapy compared with parental, chemo-sensitive cells in gene expression datasets of multiple models of chemoresistance. Further, exposure of SKOV-3 cells to cytotoxic chemotherapy causes acute, persistent downregulation of RGS10 and RGS17 transcript expression. Direct inhibition of RGS10 or RGS17 expression using siRNA knock-down significantly reduces chemotherapy-induced cell toxicity. The effects of cisplatin, vincristine, and docetaxel are inhibited following RGS10 and RGS17 knock-down in cell viability assays and phosphatidyl serine externalization assays in SKOV-3 cells and MDR-HeyA8 cells. We further show that AKT activation is higher following RGS10 knock-down and RGS 10 and RGS17 overexpression blocked LPA mediated activation of AKT, suggesting that RGS proteins may blunt AKT survival pathways.

CONCLUSIONS

Taken together, our data suggest that chemotherapy exposure triggers loss of RGS10 and RGS17 expression in ovarian cancer cells, and that loss of expression contributes to the development of chemoresistance, possibly through amplification of endogenous AKT signals. Our results establish RGS10 and RGS17 as novel regulators of cell survival and chemoresistance in ovarian cancer cells and suggest that their reduced expression may be diagnostic of chemoresistance.

Multiple mechanisms underlying troglitazone-induced mitochondrial permeability transition

Troglitazone, a thiazolidinedione class antidiabetic drug, was withdrawn from the market because of its severe idiosyncratic hepatotoxicity. It causes a mitochondrial permeability transition (MPT), which may in part contribute to its hepatotoxicity. In the present study, the mechanism of troglitazone mitochondrial toxicity was investigated in isolated rat liver mitochondria. Mitochondrial swelling induced by 10 μM troglitazone was attenuated by bromoenol lactone (BEL), an inhibitor of Ca²+-independent phospholipase A₂ (iPLA₂). In contrast, that induced by 50 μM troglitazone was exacerbated by BEL. This exacerbation was diminished by addition of 2mM glutathione, an antioxidant. Oxygen consumption by state 3 respiration in isolated mitochondria was also decreased by troglitazone, but it was not affected by BEL. Mitochondrial swelling induced by 10 μM troglitazone was completely attenuated in the absence of Ca²+ while that induced by 50 μM troglitazone was not affected. Addition of 1 μM cyclosporin A (CsA), an inhibitor of MPT pores, completely attenuated swelling induced by 10 μM troglitazone while it only partly diminished that induced by 50 μM troglitazone. Thus, the MPT induced by 10 and 50 μM troglitazone are regulated by different mechanism; the MPT induced by 10 μM troglitazone is regulated by the activation of iPLA₂ and caused by the opening of CsA-regulating MPT pores followed by accumulation of Ca²+ in mitochondria, while that induced by 50 μM troglitazone is partly regulated by reactive oxygen species and mainly caused by the opening of CsA-insensitive MPT pores.

Mitochondrial dysfunction and reduced prostaglandin synthesis in skeletal muscle of Group VIB Ca2+-independent phospholipase A2gamma-deficient mice

Group VIB Ca(2+)-independent phospholipase A(2)γ (iPLA(2)γ) is a membrane-bound iPLA(2) enzyme with unique features, such as the utilization of distinct translation initiation sites and the presence of mitochondrial and peroxisomal localization signals. Here we investigated the physiological functions of iPLA(2)γ by disrupting its gene in mice. iPLA(2)γ-knockout (KO) mice were born with an expected Mendelian ratio and appeared normal and healthy at the age of one month but began to show growth retardation from the age of two months as well as kyphosis and significant muscle weakness at the age of four months. Electron microscopy revealed swelling and reduced numbers of mitochondria and atrophy of myofilaments in iPLA(2)γ-KO skeletal muscles. Increased lipid peroxidation and the induction of several oxidative stress-related genes were also found in the iPLA(2)γ-KO muscles. These results provide evidence that impairment of iPLA(2)γ causes mitochondrial dysfunction and increased oxidative stress, leading to the loss of skeletal muscle structure and function. We further found that the compositions of cardiolipin and other phospholipid subclasses were altered and that the levels of myoprotective prostanoids were reduced in iPLA(2)γ-KO skeletal muscle. Thus, in addition to maintenance of homeostasis of the mitochondrial membrane, iPLA(2)γ may contribute to modulation of lipid mediator production in vivo.

Inhibition of calcium-independent phospholipase A2 activates p38 MAPK signaling pathways during cytostasis in prostate cancer cells

The p38 mitogen-activated protein kinase (MAPK) signaling pathways activated during cytostasis induced by Ca(2+)-independent phospholipase A2 (iPLA2) inhibition in prostate cancer cells were investigated. iPLA2 inhibition using siRNA, or the selective inhibitor bromoenol lactone (BEL) and it's enantiomers, decreased growth in LNCaP (p53 positive) and PC-3 (p53 negative) human prostate cancer cells. Decreased cell growth correlated to time- and concentration-dependent activation of the mitogen-activated protein kinase p38 in both cell lines. Inhibition of cytosolic iPLA(2)beta using S-BEL, induced significantly higher levels of P-p53, p53, p21 and P-p38 expression than inhibition of microsomal iPLA2 gamma using R-BEL. Inhibition of p38 using SB202190 or SB203580 inhibited BEL-induced increases in P-p53 (ser15), p53 and p21, and altered the number of cells in G1 in LNCaP cells, and S-phase in PC-3 cells. BEL treatment also induced reactive species in PC-3 and LNCaP cells, which was partially reversed by pretreatment with N-acetyl-cysteine (NAC). NAC subsequently inhibited BEL-induced activation of p38 and p53 in LNCaP cells. In addition, treatment of cells with NAC partially reversed the effect of BEL on cell growth and preserved cell morphology. Collectively, these data demonstrate the novel findings that iPLA2 inhibition activates p38 by inducing reactive species, and further suggest that this signaling kinase is involved in p53 activation, cell cycle arrest and cytostasis.

Genetic ablation of calcium-independent phospholipase A2{gamma} leads to alterations in hippocampal cardiolipin content and molecular species distribution, mitochondrial degeneration, autophagy, and cognitive dysfunction

Genetic ablation of calcium-independent phospholipase A(2)gamma (iPLA(2)gamma) results in profound alterations in hippocampal phospholipid metabolism and mitochondrial phospholipid homeostasis resulting in enlarged and degenerating mitochondria leading to autophagy and cognitive dysfunction. Shotgun lipidomics demonstrated multiple alterations in hippocampal lipid metabolism in iPLA(2)gamma(-/-) mice including: 1) a markedly elevated hippocampal cardiolipin content with an altered molecular species composition characterized by a shift to shorter chain length molecular species; 2) alterations in both choline and ethanolamine glycerophospholipids, including a decreased plasmenylethanolamine content; 3) increased oxidized phosphatidylethanolamine molecular species; and 4) an increased content of ceramides. Electron microscopic examination demonstrated the presence of enlarged heteromorphic lamellar structures undergoing degeneration accompanied by the presence of ubiquitin positive spheroid inclusion bodies. Purification of these enlarged heteromorphic lamellar structures by buoyant density centrifugation and subsequent SDS-PAGE and proteomics identified them as degenerating mitochondria. Collectively, these results identify the obligatory role of iPLA(2)gamma in neuronal mitochondrial lipid metabolism and membrane structure demonstrating that iPLA(2)gamma loss of function results in a mitochondrial neurodegenerative disorder characterized by degenerating mitochondria, autophagy, and cognitive dysfunction.

Suppression of mitochondrial function by oxidatively truncated phospholipids is reversible, aided by bid, and suppressed by Bcl-XL

Oxidatively truncated phospholipids are present in atherosclerotic lesions, apoptotic cells, and oxidized low density lipoproteins. Some of these lipids rapidly enter cells to induce apoptosis by the intrinsic pathway, but how such lipids initiate this process is unknown. We show the truncated phospholipid hexadecyl azelaoyl glycerophosphocholine (Az-LPAF), derived from the fragmentation of abundant sn-2 linoleoyl residues, depolarized mitochondria of intact cells. Az-LPAF also depolarized isolated mitochondria and allowed NADH loss, but did not directly interfere with complex I function. Cyclosporin A blockade of the mitochondrial permeability transition pore partially prevented the loss of electrochemical potential. Depolarization of isolated mitochondria by the truncated phospholipid was readily reversed by the addition of albumin that sequestered this lipid. Ectopic expression of the anti-apoptotic protein Bcl-X(L) in HL-60 cells reduced apoptosis by the truncated phospholipid by protecting their mitochondria. Mitochondria isolated from these cells were also protected from Az-LPAF-induced depolarization. Conversely mitochondria isolated from Bid(-/-) animals that lack this pro-apoptotic Bcl-2 family member were resistant to Az-LPAF depolarization. Addition of recombinant full-length Bid, which has phospholipid transfer activity, restored this sensitivity. Thus, phospholipid oxidation products physically interact with mitochondria to continually depolarize this organelle without permanent harm, and Bcl-2 family members modulate this interaction with full-length Bid acting as a co-factor for pro-apoptotic, oxidatively truncated phospholipids.

HL-1 mouse cardiomyocyte injury and death after simulated ischemia and reperfusion: roles of pH, Ca2+-independent phospholipase A2, and Na+/H+ exchange

The Ca(2+)-independent phospholipase A(2) VI (iPLA(2)-VI) and the Na(+)/H(+) exchanger isoform 1 (NHE1) are highly pH-sensitive proteins that exert both protective and detrimental effects in cardiac ischemia-reperfusion. Here, we investigated the role of extracellular pH (pH(o)) in ischemia-reperfusion injury and death and in regulation and function of iPLA(2)-VI and NHE1 under these conditions. HL-1 cardiomyocytes were exposed to simulated ischemia (SI; 0.5% O(2), 8 mM K(+), and 20 mM lactate) at pH(o) 6.0 and 7.4, with or without 4 or 8 h of reperfusion (SI/R). Cytochrome c release and caspase-3 activation were reduced after acidic compared with neutral SI, whereas necrotic death, estimated as glucose-6-phosphate dehydrogenase release, was similar in the two conditions. Inhibition of iPLA(2)-VI activity by bromoenol lactone (BEL) elicited cardiomyocyte necrosis during normoxia and after acidic, yet not after neutral, SI. The isoform-selective enantiomers R- and S-BEL both mimicked the effect of racemic BEL after acidic SI. In contrast, inhibition of NHE activity by EIPA had no significant effect on necrosis after SI. Both neutral and acidic SI were associated with a reversible loss of F-actin and cortactin integrity. Inhibition of iPLA(2)-VI disrupted F-actin, cortactin, and mitochondrial integrity, whereas inhibition of NHE slightly reduced stress fiber content. iPLA(2)-VIA and NHE1 mRNA levels were reduced during SI and upregulated in a pH(o)-dependent manner during SI/R. This also affected the subcellular localization of iPLA(2)-VIA. Thus, the mode of cell death and the roles and regulation of iPLA(2)-VI and NHE1 are at least in part determined by the pH(o) during SI. In addition to having clinically relevant implications, these findings can in part explain the contradictory results obtained from previous studies of iPLA(2)-VIA and NHE1 during cardiac I/R.

Role of Ca2+-independent phospholipase A2 in cell growth and signaling

Phospholipase A(2) (PLA(2)) are esterases that cleave glycerophospholipids to release fatty acids and lysophospholipids. Several studies demonstrate that PLA(2) regulate growth and signaling in several cell types. However, few of these studies have focused on Ca2+-independent phospholipase A(2) (iPLA(2) or Group VI PLA(2)). This class of PLA(2) was originally suggested to mediate phospholipid remodeling in several cell types including macrophages. As such, it was labeled as a housekeeping protein and thought not to play as significant of roles in cell growth as its older counterparts cytosolic PLA(2) (cPLA(2) or Group IV PLA(2)) and secretory PLA(2) (sPLA(2) or Groups I-III, V and IX-XIV PLA(2)). However, several recent studies demonstrate that iPLA(2) mediate cell growth, and do so by participating in signal transduction pathways that include epidermal growth factor receptors (EGFR), mitogen activated protein kinases (MAPK), mdm2, and even the tumor suppressor protein p53 and the cell cycle regulator p21. The exact mechanism by which iPLA(2) mediates these pathways are not known, but likely involve the generation of lipid signals such as arachidonic acid, lysophosphatidic acid (LPA) and lysophosphocholines (LPC). This review discusses the role of iPLA(2) in cell growth with special emphasis placed on their role in cell signaling. The putative lipid signals involved are also discussed.