Identification of hepatic peroxisomal phospholipase A(2) and characterization of arachidonic acid-containing choline glycerophospholipids in hepatic peroxisomes

Key regulator PNPLA8 drives phospholipid reprogramming induced proliferation and migration in triple-negative breast cancer

BACKGROUND

Triple-negative breast cancer (TNBC) is the most aggressive breast cancer subtype and leads to the poorest patient outcomes despite surgery and chemotherapy treatment. Exploring new molecular mechanisms of TNBC that could lead to the development of novel molecular targets are critically important for improving therapeutic options for treating TNBC.

METHODS

We sought to identify novel therapeutic targets in TNBC by combining genomic and functional studies with lipidomic analysis, which included mechanistic studies to elucidate the pathways that tie lipid profile to critical cancer cell properties. Our studies were performed in a large panel of human breast cancer cell lines and patient samples.

RESULTS

Comprehensive lipid profiling revealed that phospholipid metabolism is reprogrammed in TNBC cells. We discovered that patatin-like phospholipase domain-containing lipase 8 (PNPLA8) is overexpressed in TNBC cell lines and tissues from breast cancer patients. Silencing of PNPLA8 disrupted phospholipid metabolic reprogramming in TNBC, particularly affecting the levels of phosphatidylglycerol (PG), phosphatidylcholine (PC), lysophosphatidylcholine (LPC) and glycerophosphocholine (GPC). We showed that PNPLA8 is essential in regulating cell viability, migration and antioxidation in TNBC cells and promoted arachidonic acid and eicosanoid production, which in turn activated PI3K/Akt/Gsk3β and MAPK signaling.

CONCLUSIONS

Our study highlights PNPLA8 as key regulator of phospholipid metabolic reprogramming and malignant phenotypes in TNBC, which could be further developed as a novel molecular treatment target.

Determinants of Peroxisome Membrane Dynamics

Organelles within the cell are highly dynamic entities, requiring dramatic morphological changes to support their function and maintenance. As a result, organelle membranes are also highly dynamic, adapting to a range of topologies as the organelle changes shape. In particular, peroxisomes—small, ubiquitous organelles involved in lipid metabolism and reactive oxygen species homeostasis—display a striking plasticity, for example, during the growth and division process by which they proliferate. During this process, the membrane of an existing peroxisome elongates to form a tubule, which then constricts and ultimately undergoes scission to generate new peroxisomes. Dysfunction of this plasticity leads to diseases with developmental and neurological phenotypes, highlighting the importance of peroxisome dynamics for healthy cell function. What controls the dynamics of peroxisomal membranes, and how this influences the dynamics of the peroxisomes themselves, is just beginning to be understood. In this review, we consider how the composition, biophysical properties, and protein-lipid interactions of peroxisomal membranes impacts on their dynamics, and in turn on the biogenesis and function of peroxisomes. In particular, we focus on the effect of the peroxin PEX11 on the peroxisome membrane, and its function as a major regulator of growth and division. Understanding the roles and regulation of peroxisomal membrane dynamics necessitates a multidisciplinary approach, encompassing knowledge across a range of model species and a number of fields including lipid biochemistry, biophysics and computational biology. Here, we present an integrated overview of our current understanding of the determinants of peroxisome membrane dynamics, and reflect on the outstanding questions still remaining to be solved.

Protein pI and Intracellular Localization

The protein isoelectric point (pI) can be calculated from an amino acid sequence using computational analysis in a good agreement with experimental data. Availability of whole-genome sequences empowers comparative studies of proteome-wide pI distributions. It was found that the whole-proteome distributions of protein pI values are multimodal in different species. It was further hypothesized that the observed multimodality is associated with subcellular localization-specific differences in local pI distributions. Here, we overview the multimodality of proteome-wide pI distributions in different organisms focusing on the relationships between protein pI and subcellular localization. We also discuss the probable factors responsible for variation of the intracellular localization-specific pI profiles.

Functional metabolomics using UPLC-Q/TOF-MS combined with ingenuity pathway analysis as a promising strategy for evaluating the efficacy and discovering amino acid metabolism as a potential therapeutic mechanism-related target for geniposide against alcoholic liver disease

Metabolomics has been used as a strategy to evaluate the efficacy of and potential targets for natural products.

Localization-specific distributions of protein pI in human proteome are governed by local pH and membrane charge

Background

Whole-proteome distributions of protein isoelectric point (pI) values in different organisms are bi- or trimodal with some variations. It was suggested that the observed multimodality of the proteome-wide pI distributions is associated with subcellular localization-specific differences in the local pI distributions. However, the factors responsible for variation of the intracellular localization-specific pI profiles have not been investigated in detail.

Results

In this work, we explored proteome-wide pI distributions of 32,138 human proteins predicted to reside in 10 subcellular compartments, as well as the pI distributions of experimentally observed lysosomal and Golgi proteins. The distributions were found to differ significantly, although all of them adhered to the major recurrent bimodal pattern. Grossly, acid-biased and alkaline-biased patterns with various minor statistical features were observed at different subcellular locations. Bioinformatics analysis revealed the existence of strong statistically significant correlations between protein pI and subcellular localization. Most markedly, protein pI was found to correlate positively with nuclear and mitochondrial locations and negatively with cytoskeletal, cytoplasmic, lysosomal and peroxisomal environment. Further analysis demonstrated that subcellular compartment-specific pI distributions are greatly influenced by local pH and organelle membrane charge. Multiple nonlinear regression analysis identified a polynomial function of the two variables that best fitted the mean pI values of the localization-specific pI distributions. A high coefficient of determination calculated for this regression (R² = 0.98) suggests that local pH and organelle membrane charge are the major factors responsible for variation of the intracellular localization-specific pI profiles.

Conclusions

Our study demonstrates that strong correlations exist between protein pI and subcellular localization. The specific pI distributions at different subcellular locations are defined by local environment. Predominantly, it is the local pH and membrane charge that shape the organelle-specific protein pI patterns. These findings expand our understanding of spatial organization of the human proteome.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0221-4) contains supplementary material, which is available to authorized users.

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.

Plasmalogen homeostasis - regulation of plasmalogen biosynthesis and its physiological consequence in mammals

Plasmalogens, mostly ethanolamine-containing alkenyl ether phospholipids, are a major subclass of glycerophospholipids. Plasmalogen synthesis is initiated in peroxisomes and completed in the endoplasmic reticulum. The absence of plasmalogens in several organs of peroxisome biogenesis-defective patients suggests that the de novo synthesis of plasmalogens plays a pivotal role in its homeostasis in tissues. Plasmalogen synthesis is regulated by modulating the stability of fatty acyl-CoA reductase 1 on peroxisomal membranes, a rate-limiting enzyme in plasmalogen synthesis, by sensing plasmalogens in the inner leaflet of plasma membranes. Dysregulation of plasmalogen homeostasis impairs cholesterol biosynthesis by altering the stability of squalene monooxygenase, a key enzyme in cholesterol biosynthesis, implying physiological consequences of plasmalogen homeostasis with respect to cholesterol metabolism in cells, as well as in organs such as the liver.

Plasmalogen biosynthesis is spatiotemporally regulated by sensing plasmalogens in the inner leaflet of plasma membranes

Alkenyl ether phospholipids are a major sub-class of ethanolamine- and choline-phospholipids in which a long chain fatty alcohol is attached at the sn-1 position through a vinyl ether bond. Biosynthesis of ethanolamine-containing alkenyl ether phospholipids, plasmalogens, is regulated by modulating the stability of fatty acyl-CoA reductase 1 (Far1) in a manner dependent on the level of cellular plasmalogens. However, precise molecular mechanisms underlying the regulation of plasmalogen synthesis remain poorly understood. Here we show that degradation of Far1 is accelerated by inhibiting dynamin-, Src kinase-, or flotillin-1-mediated endocytosis without increasing the cellular level of plasmalogens. By contrast, Far1 is stabilized by sequestering cholesterol with nystatin. Moreover, abrogation of the asymmetric distribution of plasmalogens in the plasma membrane by reducing the expression of CDC50A encoding a β-subunit of flippase elevates the expression level of Far1 and plasmalogen synthesis without reducing the total cellular level of plasmalogens. Together, these results support a model that plasmalogens localised in the inner leaflet of the plasma membranes are sensed for plasmalogen homeostasis in cells, thereby suggesting that plasmalogen synthesis is spatiotemporally regulated by monitoring cellular level of plasmalogens.

Proteome analysis of peroxisomes from etiolated Arabidopsis seedlings identifies a peroxisomal protease involved in β-oxidation and development

Plant peroxisomes are highly dynamic organelles that mediate a suite of metabolic processes crucial to development. Peroxisomes in seeds/dark-grown seedlings and in photosynthetic tissues constitute two major subtypes of plant peroxisomes, which had been postulated to contain distinct primary biochemical properties. Multiple in-depth proteomic analyses had been performed on leaf peroxisomes, yet the major makeup of peroxisomes in seeds or dark-grown seedlings remained unclear. To compare the metabolic pathways of the two dominant plant peroxisomal subtypes and discover new peroxisomal proteins that function specifically during seed germination, we performed proteomic analysis of peroxisomes from etiolated Arabidopsis (Arabidopsis thaliana) seedlings. The detection of 77 peroxisomal proteins allowed us to perform comparative analysis with the peroxisomal proteome of green leaves, which revealed a large overlap between these two primary peroxisomal variants. Subcellular targeting analysis by fluorescence microscopy validated around 10 new peroxisomal proteins in Arabidopsis. Mutant analysis suggested the role of the cysteine protease RESPONSE TO DROUGHT21A-LIKE1 in β-oxidation, seed germination, and growth. This work provides a much-needed road map of a major type of plant peroxisome and has established a basis for future investigations of peroxisomal proteolytic processes to understand their roles in development and in plant interaction with the environment.

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.

Docosahexaenoic acid mediates peroxisomal elongation, a prerequisite for peroxisome division

Peroxisome division is regulated by several factors, termed fission factors, as well as the conditions of the cellular environment. Over the past decade, the idea of metabolic control of peroxisomal morphogenesis has been postulated, but remains largely undefined to date. In the current study, docosahexaenoic acid (DHA, C22:6n-3) was identified as an inducer of peroxisome division. In fibroblasts isolated from patients that carry defects in peroxisomal fatty acid β-oxidation, peroxisomes are much less abundant than normal cells. Treatment of these patient fibroblasts with DHA induced the proliferation of peroxisomes to the level seen in normal fibroblasts. DHA-induced peroxisomal proliferation was abrogated by treatment with a small inhibitory RNA (siRNA) targeting dynamin-like protein 1 and with dynasore, an inhibitor of dynamin-like protein 1, which suggested that DHA stimulates peroxisome division. DHA augmented the hyper-oligomerization of Pex11pβ and the formation of Pex11pβ-enriched regions on elongated peroxisomes. Time-lapse imaging analysis of peroxisomal morphogenesis revealed a sequence of steps involved in peroxisome division, including elongation in one direction followed by peroxisomal fission. DHA enhanced peroxisomal division in a microtubule-independent manner. These results suggest that DHA is a crucial signal for peroxisomal elongation, a prerequisite for subsequent fission and peroxisome division.

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.

Transfer of metabolites across the peroxisomal membrane

Peroxisomes perform a large variety of metabolic functions that require a constant flow of metabolites across the membranes of these organelles. Over the last few years it has become clear that the transport machinery of the peroxisomal membrane is a unique biological entity since it includes nonselective channels conducting small solutes side by side with transporters for 'bulky' solutes such as ATP. Electrophysiological experiments revealed several channel-forming activities in preparations of plant, mammalian, and yeast peroxisomes and in glycosomes of Trypanosoma brucei. The properties of the first discovered peroxisomal membrane channel - mammalian Pxmp2 protein - have also been characterized. The channels are apparently involved in the formation of peroxisomal shuttle systems and in the transmembrane transfer of various water-soluble metabolites including products of peroxisomal β-oxidation. These products are processed by a large set of peroxisomal enzymes including carnitine acyltransferases, enzymes involved in the synthesis of ketone bodies, thioesterases, and others. This review discusses recent data pertaining to solute permeability and metabolite transport systems in peroxisomal membranes and also addresses mechanisms responsible for the transfer of ATP and cofactors such as an ATP transporter and nudix hydrolases.

ADP ribosylation factors 1 and 4 and group VIA phospholipase A₂ regulate morphology and intraorganellar traffic in the endoplasmic reticulum-Golgi intermediate compartment

In search of morphological determinants for the endoplasmic reticulum-Golgi intermediate compartment (ERGIC), we found that a concerted action of Arf1, Arf4, and PLA2G6-A controls the architecture of the ERGIC by regulating tubular carriers. This is predicted to impact the rate of transport and destination of cargos in the ERGIC.

Organelle morphology of the endomembrane system is critical for optimal organelle function. ADP ribosylation factors (Arfs), a family of small GTPases, are required for maintaining the structure of the Golgi and endosomes. What determines the discontinuous nature of the endoplasmic reticulum (ER)–Golgi intermediate compartment (ERGIC) as tubulovesicular clusters is unknown. In search of morphological determinants for the ERGIC, we found that a double knockdown of Arf1+Arf4 induced dynamic ERGIC tubules that connect ERGIC clusters, indicating that the tubules mediated lateral intraERGIC traffic. Tubule formation was inhibited by an antagonist of group VI calcium-independent phospholipase A₂ (PLA2G6) and by silencing the A isoform of PLA2G6 (PLA2G6-A). Arf1+Arf4 depletion altered the expression of PLA2G6-A splice variants and relocalized PLA2G6-A from the cytosol to ERGIC clusters and tubules, suggesting that the enzyme became locally active. We show that changes in Arf1 can modulate the activity of PLA2G6-A. We propose that a concerted action of Arf1, Arf4, and PLA2G6-A controls the architecture of the ERGIC in a way that is predicted to impact the rate and possibly the destination of cargos. Our findings have identified key components in the molecular mechanism underlying the regulation of tubules in the ERGIC and uncover tubular carriers as tightly controlled machinery.

Group VIA Ca2+-independent phospholipase A2 (iPLA2beta) and its role in beta-cell programmed cell death

Activation of phospholipases A(2) (PLA(2)s) leads to the generation of biologically active lipid mediators that can affect numerous cellular events. The Group VIA Ca(2+)-independent PLA(2), designated iPLA(2)beta, is active in the absence of Ca(2+), activated by ATP, and inhibited by the bromoenol lactone suicide inhibitor (BEL). Over the past 10-15 years, studies using BEL have demonstrated that iPLA(2)beta participates in various biological processes and the recent availability of mice in which iPLA(2)beta expression levels have been genetically-modified are extending these findings. Work in our laboratory suggests that iPLA(2)beta activates a unique signaling cascade that promotes beta-cell apoptosis. This pathway involves iPLA(2)beta dependent induction of neutral sphingomyelinase, production of ceramide, and activation of the intrinsic pathway of apoptosis. There is a growing body of literature supporting beta-cell apoptosis as a major contributor to the loss of beta-cell mass associated with the onset and progression of Type 1 and Type 2 diabetes mellitus. This underscores a need to gain a better understanding of the molecular mechanisms underlying beta-cell apoptosis so that improved treatments can be developed to prevent or delay the onset and progression of diabetes mellitus. Herein, we offer a general review of Group VIA Ca(2+)-independent PLA(2) (iPLA(2)beta) followed by a more focused discussion of its participation in beta-cell apoptosis. We suggest that iPLA(2)beta-derived products trigger pathways which can lead to beta-cell apoptosis during the development of diabetes.

Posttranslational regulation of fatty acyl-CoA reductase 1, Far1, controls ether glycerophospholipid synthesis

Plasmalogens are a major subclass of ethanolamine and choline glycerophospholipids in which a long chain fatty alcohol is attached at the sn-1 position through a vinyl ether bond. This ether-linked alkyl bond is formed in peroxisomes by replacement of a fatty acyl chain in the intermediate 1-acyl-dihydroxyacetone phosphate with a fatty alcohol in a reaction catalyzed by alkyl dihydroxyacetone phosphate synthase. Here, we demonstrate that the enzyme fatty acyl-CoA reductase 1 (Far1) supplies the fatty alcohols used in the formation of ether-linked alkyl bonds. Far1 activity is elevated in plasmalogen-deficient cells, and conversely, the levels of this enzyme are restored to normal upon plasmalogen supplementation. Down-regulation of Far1 activity in response to plasmalogens is achieved by increasing the rate of degradation of peroxisomal Far1 protein. Supplementation of normal cells with ethanolamine and 1-O-hexadecylglycerol, which are intermediates in plasmalogen biosynthesis, accelerates degradation of Far1. Taken together, our results indicate that ether lipid biosynthesis in mammalian cells is regulated by a negative feedback mechanism that senses cellular plasmalogen levels and appropriately increases or decreases Far1.

Tryptase activates calcium-independent phospholipase A2 and releases PGE2 in airway epithelial cells

Human small airway epithelial cells (HSAEC) form the boundary between the external environmental allergens and the internal lung milieu. Mast cells are present in human lung tissue interspersed within the pulmonary epithelium and can secrete a host of pre- and newly formed mediators from their granules, which may propagate small airway inflammation. In this study, tryptase stimulation of HSAEC increased membrane-associated, calcium-independent phospholipase A(2)gamma (iPLA(2)gamma) activity, resulting in increased arachidonic acid and PGE(2) release. These responses were inhibited by pretreating HSAEC with the iPLA(2)-selective inhibitor bromoenol lactone. The tryptase-stimulated PGE(2) production was inhibited by treating HSAEC with the cyclooxygenase (COX)-1-selective inhibitor SC-560 and the nonselective COX inhibitor aspirin but not by the COX-2-selective inhibitor CAY10404, indicating that the early release of arachidonic acid is metabolized by constitutive COX-1 to form PGE(2) in tryptase-stimulated HSAEC. Additionally, platelet-activating factor production and neutrophil adherence to tryptase-stimulated HSAEC was also increased. This complex response can set up a cascade of inflammatory mediator production in small airways. We speculate that selective inhibition of iPLA(2)gamma-mediated phospholipid hydrolysis may prove beneficial in inflammatory airway diseases.

Calcium-independent phospholipase A2 in rabbit ventricular myocytes

We have previously reported that the majority of phospholipase A2 (PLA2) activity in rabbit ventricular myocytes is membrane-associated, calcium-independent (iPLA2), selective for arachidonylated plasmalogen phospholipids and inhibited by the iPLA2-selective inhibitor bromoenol lactone (BEL). Here, we identified the presence of iPLA2 in rabbit ventricular myocytes, determined the full length sequences for rabbit iPLA2beta and iPLA2gamma and compared their homology to the human isoforms. Rabbit iPLA2beta encoded a protein with a predicated molecular mass of 74 kDa that is 91% identical to the human iPLA2beta short isoform. Full length iPLA2gamma protein has a predicated molecular mass of 88 kDa and is 88% identical to the human isoform. Immunoblot analysis of iPLA2beta and gamma in membrane and cytosolic fractions from rabbit and human cardiac myocytes demonstrated a similar pattern of distribution with both isoforms present in the membrane fraction, but no detectable protein in the cytosol. Membrane-associated iPLA2 activity was inhibited preferentially by the R enantiomer of bromoenol lactone [(R)-BEL], indicating that the majority of activity is due to iPLA2gamma.

Role of calcium-independent phospholipase A2 in complement-mediated glomerular epithelial cell injury

In experimental membranous nephropathy, complement C5b-9-induced glomerular epithelial cell (GEC) injury leads to morphological changes in GEC and proteinuria, in association with phospholipase A2 (PLA2) activation. The present study addresses the role of calcium-independent PLA2 (iPLA2) in GEC injury. iPLA2β short and iPLA2γ were expressed in cultured rat GEC and normal rat glomeruli. To determine whether iPLA2 is involved in complement-mediated arachidonic acid (AA) release, GEC were stably transfected with iPLA2γ or iPLA2β cDNAs (GEC-iPLA2γ; GEC-iPLA2β). Compared with control cells (GEC-Neo), GEC-iPLA2γ and GEC-iPLA2β demonstrated greater expression of iPLA2 proteins and activities. Complement-mediated release of [3H]AA was augmented significantly in GEC-iPLA2γ compared with GEC-Neo, and the augmented [3H]AA release was inhibited by the iPLA2-directed inhibitor bromoenol lactone (BEL). For comparison, overexpression of iPLA2γ also amplified [3H]AA release after incubation of GEC with H2O2, or chemical anoxia followed by reexposure to glucose (in vitro ischemia-reperfusion injury). In parallel with release of [3H]AA, complement-mediated production of prostaglandin E2 was amplified in GEC-iPLA2γ. Complement-mediated cytotoxicity was attenuated significantly in GEC-iPLA2γ compared with GEC-Neo, and the cytoprotective effect of iPLA2γ was reversed by BEL, and in part by indomethacin. Overexpression of iPLA2β did not amplify complement-dependent [3H]AA release, but nonetheless attenuated complement-mediated cytotoxicity. Thus iPLA2γ may be involved in complement-mediated release of AA. Expression of iPLA2γ or iPLA2β induces cytoprotection against complement-dependent GEC injury. Modulation of iPLA2 activity may prove to be a novel approach to reducing GEC injury.

A novel role of group VIB calcium-independent phospholipase A2 (iPLA2gamma) in the inducible expression of group IIA secretory PLA2 in rat fibroblastic cells

Group IIA secretory phospholipase A(2) (sPLA(2)-IIA) is a prototypic sPLA(2) enzyme that may play roles in modification of eicosanoid biosynthesis as well as antibacterial defense. In several cell types, inducible expression of sPLA(2) by pro-inflammatory stimuli is attenuated by group IVA cytosolic PLA(2) (cPLA(2)alpha) inhibitors such as arachidonyl trifluoromethyl ketone, leading to the proposal that prior activation of cPLA(2)alpha is required for de novo induction of sPLA(2). However, because of the broad specificity of several cPLA(2)alpha inhibitors used so far, a more comprehensive approach is needed to evaluate the relevance of this ambiguous pathway. Here, we provide evidence that the induction of sPLA(2)-IIA by pro-inflammatory stimuli requires group VIB calcium-independent PLA(2) (iPLA(2)gamma), rather than cPLA(2)alpha, in rat fibroblastic 3Y1 cells. Results with small interfering RNA unexpectedly showed that the cytokine induction of sPLA(2)-IIA in cPLA(2)alpha knockdown cells, in which cPLA(2)alpha protein was undetectable, was similar to that in replicate control cells. By contrast, knockdown of iPLA(2)gamma, another arachidonyl trifluoromethyl ketone-sensitive intracellular PLA(2), markedly reduced the cytokine-induced expression of sPLA(2)-IIA. Supporting this finding, the R-enantiomer of bromoenol lactone, an iPLA(2)gamma inhibitor, suppressed the cytokine-induced sPLA(2)-IIA expression, whereas (S)-bromoenol lactone, an iPLA(2)beta inhibitor, failed to do so. Moreover, lipopolysaccharide-stimulated sPLA(2)-IIA expression was also abolished by knockdown of iPLA(2)gamma. These findings open new insight into a novel regulatory role of iPLA(2)gamma in stimulus-coupled sPLA(2)-IIA expression.

Dramatic Accumulation of Triglycerides and Precipitation of Cardiac Hemodynamic Dysfunction during Brief Caloric Restriction in Transgenic Myocardium Expressing Human Calcium-independent Phospholipase A2γ

Previously, we identified calcium-independent phospholipase A2gamma (iPLA2gamma) with multiple translation initiation sites and dual mitochondrial and peroxisomal localization motifs. To determine the role of iPLA2gamma in integrating lipid and energy metabolism, we generated transgenic mice containing the alpha-myosin heavy chain promoter (alphaMHC) placed proximally to the human iPLA2gamma coding sequence that resulted in cardiac myocyte-restricted expression of iPLA2gamma (TGiPLA2gamma). TGiPLA2gamma mice possessed multiple phenotypes including: 1) a dramatic approximately 35% reduction in myocardial phospholipid mass in both the fed and mildly fasted states; 2) a marked accumulation of triglycerides during brief caloric restriction that represented 50% of total myocardial lipid mass; and 3) acute fasting-induced hemodynamic dysfunction. Biochemical characterization of the TGiPLA2gamma protein expressed in cardiac myocytes demonstrated over 25 distinct isoforms by two-dimensional SDS-PAGE Western analysis. Immunohistochemistry identified iPLA2gamma in the peroxisomal and mitochondrial compartments in both wild type and transgenic myocardium. Electron microscopy revealed the presence of loosely packed and disorganized mitochondrial cristae in TGiPLA2gamma mice that were accompanied by defects in mitochondrial function. Moreover, markedly elevated levels of 1-hydroxyl-2-arachidonoyl-sn-glycero-3-phosphocholine and 1-hydroxyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine were prominent in the TGiPLA2gamma myocardium identifying the production of signaling metabolites by this enzyme in vivo. Collectively, these results identified the participation of iPLA2gamma in the remarkable lipid plasticity of myocardium, its role in generating signaling metabolites, and its prominent effects in modulating energy storage and utilization in myocardium in different metabolic contexts.

Peroxisomal membrane permeability and solute transfer

The review is dedicated to recent progress in the study of peroxisomal membrane permeability to solutes which has been a matter of debate for more than 40 years. Apparently, the mammalian peroxisomal membrane is freely permeable to small solute molecules owing to the presence of pore-forming channels. However, the membrane forms a permeability barrier for 'bulky' solutes including cofactors (NAD/H, NADP/H, CoA, and acetyl/acyl-CoA esters) and ATP. Therefore, peroxisomes need specific protein transporters to transfer these compounds across the membrane. Recent electrophysiological studies have revealed channel-forming activities in the mammalian peroxisomal membrane. The possible involvement of the channels in the transfer of small metabolites and in the formation of peroxisomal shuttle systems is described.

Phospholipase A2-catalyzed hydrolysis of plasmalogen phospholipids in thrombin-stimulated human platelets

In the present study, phospholipase A(2) (PLA(2))-catalyzed hydrolysis of platelet membrane phospholipids was investigated by measuring PLA(2) activity, phospholipid hydrolysis, arachidonic acid release and choline lysophospholipid production in thrombin-stimulated human platelets. Thrombin-stimulated platelets demonstrated selective hydrolysis of arachidonylated plasmenylcholine and plasmenylethanolamine, with little change in diacyl phospholipids. Accelerated plasmalogen hydrolysis was accompanied by increased arachidonic acid and thromboxane B(2) release and increased lysoplasmenylcholine production. Thrombin stimulation caused an increase in PLA(2) activity measured in the cytosolic fraction with plasmenylcholine only; no increase in activity was measured with phosphatidylcholine. No change in membrane-associated PLA(2) activity was observed with either substrate tested. Pretreatment with the Ca(2+)-independent PLA(2)-selective inhibitor, bromoenol lactone, inhibited completely any thrombin-stimulated phospholipid hydrolysis. Thus, thrombin stimulation of human platelets activates a cytosolic PLA(2) that selectively hydrolyzes arachidonylated plasmalogen phospholipids.

Identification of calcium-independent phospholipase A2gamma in mitochondria and its role in mitochondrial oxidative stress

Oxidant-induced lipid peroxidation and cell death mediate pathologies associated with ischemia-reperfusion and inflammation. Our previous work in rabbit renal proximal tubular cells (RPTC) demonstrated that inhibition of Ca(2+)-independent phospholipase A(2) (iPLA(2)) potentiates oxidant-induced lipid peroxidation and necrosis, implicating iPLA(2) in phospholipid repair. This study was conducted to identify a RPTC mitochondrial PLA(2) and determine the role of PLA(2) in oxidant-induced mitochondrial dysfunction. iPLA(2) activity was detected in Percoll-purified rabbit renal cortex mitochondria (RCM) and in isolated mitochondrial inner membrane fractions from rabbit and human RCM. Immunoblot analysis and inhibitor sensitivity profiles revealed that iPLA(2)gamma is the RCM iPLA(2) activity. RCM iPLA(2) activity was enhanced in the presence of ATP and was blocked by the PKCepsilon V1-2 inhibitor. Oxidant-induced mitochondrial lipid peroxidation and swelling were accelerated by pretreatment with R-BEL, but not S-BEL. Furthermore, oxidant treatment of isolated RCM resulted in decreased iPLA(2)gamma activity. These results reveal that RCM iPLA(2) is iPLA(2)gamma, RCM iPLA(2)gamma is regulated by phosphorylation by PKCepsilon, iPLA(2)gamma protects RCM from oxidant-induced lipid peroxidation and dysfunction, and that a strategy to preserve or enhance iPLA(2)gamma activity may be of therapeutic benefit.

Effects of biological oxidants on the catalytic activity and structure of group VIA phospholipase A2

Group VIA phospholipase A(2) (iPLA(2)beta) is expressed in phagocytes, vascular cells, pancreatic islet beta-cells, neurons, and other cells and plays roles in transcriptional regulation, cell proliferation, apoptosis, secretion, and other events. A bromoenol lactone (BEL) suicide substrate used to study iPLA(2)beta functions inactivates iPLA(2)beta by alkylating Cys thiols. Because thiol redox reactions are important in signaling and some cells that express iPLA(2)beta produce biological oxidants, iPLA(2)beta might be subject to redox regulation. We report that biological concentrations of H(2)O(2), NO, and HOCl inactivate iPLA(2)beta, and this can be partially reversed by dithiothreitol (DTT). Oxidant-treated iPLA(2)beta modifications were studied by LC-MS/MS analyses of tryptic digests and included DTT-reversible events, e.g., formation of disulfide bonds and sulfenic acids, and others not so reversed, e.g., formation of sulfonic acids, Trp oxides, and Met sulfoxides. W(460) oxidation could cause irreversible inactivation because it is near the lipase consensus sequence ((463)GTSTG(467)), and site-directed mutagenesis of W(460) yields active mutant enzymes that exhibit no DTT-irreversible oxidative inactivation. Cys651-sulfenic acid formation could be one DTT-reversible inactivation event because Cys651 modification correlates closely with activity loss and its mutagenesis reduces sensitivity to inhibition. Intermolecular disulfide bond formation might also cause reversible inactivation because oxidant-treated iPLA(2)beta contains DTT-reducible oligomers, and oligomerization occurs with time- and temperature-dependent iPLA(2)beta inactivation that is attenuated by DTT or ATP. Subjecting insulinoma cells to oxidative stress induces iPLA(2)beta oligomerization, loss of activity, and subcellular redistribution and reduces the rate of release of arachidonate from phospholipids. These findings raise the possibility that redox reactions affect iPLA(2)beta functions.

Identification of Proteomic Signatures of Exposure to Marine Pollutants in Mussels (Mytilus edulis)

Bivalves and especially mussels are very good indicators of marine and estuarine pollution, and so they have been widely used in biomonitoring programs all around the world. However, traditional single parameter biomarkers face the problem of high sensitivity to biotic and abiotic factors. In our study, digestive gland peroxisome-enriched fractions of Mytilus edulis (L., 1758) were analyzed by DIGE and MS. We identified several proteomic signatures associated with the exposure to several marine pollutants (diallyl phthalate, PBDE-47, and bisphenol-A). Animals collected from North Atlantic Sea were exposed to the contaminants independently under controlled laboratory conditions. One hundred and eleven spots showed a significant increase or decrease in protein abundance in the two-dimensional electrophoresis maps from the groups exposed to pollutants. We obtained a unique protein expression signature of exposure to each of those chemical compounds. Moreover a set of proteins composed a proteomic signature in common to the three independent exposures. It is remarkable that the principal component analysis of these spots showed a discernible separation between groups, and so did the hierarchical clustering into four classes. The 14 proteins identified by MS participate in alpha- and beta-oxidation pathways, xenobiotic and amino acid metabolism, cell signaling, oxyradical metabolism, peroxisomal assembly, respiration, and the cytoskeleton. Our results suggest that proteomic signatures could become a valuable tool to monitor the presence of pollutants in field experiments where a mixture of pollutants is often present. Further studies on the identified proteins could provide crucial information to understand possible mechanisms of toxicity of single xenobiotics or mixtures of them in marine ecosystems.

Differential Roles for Cytosolic and Microsomal Ca2+-Independent Phospholipase A2in Cell Growth and Maintenance of Phospholipids

Physiological roles of microsomal (iPLA(2)gamma) and cytosolic (iPLA(2)beta)Ca(2+)-independent phospholipase A(2) were determined in two different epithelial cell models. R- and S-enantiomers of the iPLA(2) inhibitor bromoenol lactone (BEL) were isolated and shown to selectively inhibit iPLA(2gamma) and iPLA(2beta), respectively. The effect of these enantiomers on cell growth was assessed in human embryonic kidney 293 and Caki-1 cells using 3-(4-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT). S-BEL (0-5.0 microM) decreased MTT staining 35% after 24 h compared with control cells, whereas treatment with either R-BEL or R/S-BEL induced 15% decreases. Neither R-BEL nor S-BEL induced cell death as determined by annexin V and propidium iodide staining. Transfection of cells with iPLA(2)beta siRNA reduced MTT staining approximately 35%, whereas transfection of cells with iPLA(2)gamma siRNA only decreased MTT staining 10 to 15% compared with control cells. The effect of iPLA(2)beta and iPLA(2)gamma siRNA on cell number and protein was also determined, and iPLA(2)beta siRNA decreased cell number and protein 25% compared with control cells. In contrast, iPLA(2)gamma siRNA decreased cell number, but not cellular protein, compared with control cells. Selective inhibition of iPLA(2)beta, but not iPLA(2)gamma, decreased several arachidonic acid-containing phospholipids, including 16:1-20:4, 16:0-20:4, 18:1-20:4, and 18:0-20:4 phosphatidylcholine, showing that the ability of iPLA(2)beta inhibitors to decrease cell growth correlates with their ability to decrease arachidonic acid-containing phospholipids. These data show that iPLA(2)beta inhibition results in greater decreases in cell growth and proliferation than iPLA(2)gamma, identifies specific phospholipids whose expressions are differentially regulated by iPLA(2)beta and iPLA(2)gamma, and suggests novel roles for iPLA(2)beta in cell growth.

A bromoenol lactone suicide substrate inactivates group VIA phospholipase A2 by generating a diffusible bromomethyl keto acid that alkylates cysteine thiols

Phospholipases A2 (PLA2) comprise a superfamily of enzymes that hydrolyze phospholipids to a free fatty acid, e.g., arachidonate, and a 2-lysophospholipid. Dissecting their individual functions has relied in large part on pharmacological inhibitors that discriminate among PLA2. Group VIA PLA2 (iPLA2beta) has a GTSTG serine lipase consensus sequence, and studies with a bromoenol lactone (BEL) suicide substrate inhibitor have been taken to suggest that iPLA2beta participates in a wide variety of biological processes. Such conclusions presume inhibitor specificity. Inhibition by BEL requires its hydrolysis by and results in uncharacterized covalent modification(s) of iPLA2beta. We performed mass spectrometric analyses of proteolytic digests of BEL-treated iPLA2beta to identify modifications associated with loss of activity. The GTSTG active site and large flanking regions of sequence are not modified by BEL treatment, but most iPLA2beta Cys residues are alkylated at various BEL concentrations to form a thioether linkage to a BEL keto acid hydrolysis product. Synthetic Cys-containing peptides are alkylated when incubated with iPLA2beta and BEL, which reflects iPLA2beta-catalyzed BEL hydrolysis to a diffusible bromomethyl keto acid product that reacts with distant thiols. The BEL concentration dependence of Cys651 alkylation closely parallels that of loss of iPLA2beta activity. No amino acid residues other than Cys were found to be modified, suggesting that Cys alkylation is the covalent modification of iPLA2beta responsible for loss of activity, and the alkylating species appears to be a diffusible hydrolysis product of BEL rather than a tethered acyl-enzyme intermediate.

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The effect of inhibition of Ca2+-independent phospholipase A2 on chemotherapeutic-induced death and phospholipid profiles in renal cells

We demonstrate that cells derived from primary cultures of rabbit proximal tubules (RPTC), human embryonic kidney (HEK293) and human kidney carcinomas (Caki-1) express microsomal Ca(2+)-independent phospholipase A(2) (iPLA(2)gamma) and cytosolic Ca(2+)-independent phospholipase A(2) (iPLA(2)beta). Inhibition of iPLA(2) activity in these cells using the iPLA(2) inhibitor bromoenol lactone (BEL) (0-5.0microM) for 24h did not induce cell death as determined by annexin V and propidium iodide (PI) staining. However, BEL treatment prior to cisplatin (50muM) or vincristine (2microM) exposure reduced apoptosis 30-50% in all cells tested (RPTC, HEK293 and Caki-1 cells). To identify the phospholipids altered during cell death electrospray ionization-mass spectrometry and lipidomic analysis of HEK293 and Caki-1 cells was performed. Cisplatin treatment reduced 14:0-16:0 and 16:0-16:0 phosphatidylcholine (PtdCho) 50% and 35%, respectively, in both cell lines, 16:0-18:2 PtdCho in Caki-1 cells and increased 16:1-22:6 plasmenylcholine (PlsCho). BEL treatment prior to cisplatin exposure further decreased 14:0-16:0 PtdCho, 16:0-16:1 PlsCho and 16:0-18:1 PlsCho in HEK293 cells, and inhibited cisplatin-induced increases in 16:1-22:6 PlsCho in Caki-1 cells. Treatment of cells with BEL prior to cisplatin exposure also increased the levels of several arachidonic containing phospholipids including 16:0-20:4, 18:1-20:4, and 18:0-20:4 PtdCho, compared to cisplatin only treated cells. These data demonstrate that inhibition of iPLA(2) protects against chemotherapeutic-induced cell death in multiple human renal cell models, identifies specific phospholipids whose levels are altered during cell death, and demonstrates that alterations in these phospholipids correlate to the protection against cell death in the presence of iPLA(2) inhibitors.

Complex transcriptional and translational regulation of iPLAgamma resulting in multiple gene products containing dual competing sites for mitochondrial or peroxisomal localization

Membrane-associated calcium-independent phospholipase A2gamma (iPLA2gamma) contains four potential in-frame methionine start sites (Mancuso, D.J. Jenkins, C.M. & Gross, R.W. (2000) J. Biol. Chem.275, 9937-9945), but the mechanisms regulating the types, amount and subcellular localization of iPLA2gamma in cells are incompletely understood. We now: (a) demonstrate the dramatic transcriptional repression of mRNA synthesis encoding iPLA2gamma by a nucleotide sequence nested in the coding sequence itself; (b) localize the site of transcriptional repression to the most 5' sequence encoding the iPLA2gamma holoprotein; (c) identify the presence of nuclear protein constituents which bind to the repressor region by gel shift analysis; (d) demonstrate the translational regulation of distinct iPLA2gamma isoforms; (e) identify multiple novel exons, promoters, and alternative splice variants of human iPLA2gamma; (f) document the presence of dual-competing subcellular localization signals in discrete isoforms of iPLA2gamma; and (g) demonstrate the functional integrity of an N-terminal mitochondrial localization signal by fluorescence imaging and the presence of iPLA2gamma in the mitochondrial compartment of rat myocardium. The intricacy of the regulatory mechanisms of iPLA2gamma biosynthesis in rat myocardium is underscored by the identification of seven distinct protein products that utilize multiple mechanisms (transcription, translation and proteolysis) to produce discrete iPLA2gamma polypeptides containing either single or dual subcellular localization signals. This unanticipated complex interplay between peroxisomes and mitochondria mediated by competition for uptake of the nascent iPLA2gamma polypeptide identifies a new level of phospholipase-mediated metabolic regulation. Because uncoupling protein function is regulated by free fatty acids in mitochondria, these results suggest that iPLA2gamma processing contributes to integrating respiration and thermogenesis in mitochondria.

The Highly Selective Production of 2-Arachidonoyl Lysophosphatidylcholine Catalyzed by Purified Calcium-independent Phospholipase A2γ

Herein, we report the heterologous expression of the human peroxisomal 63-kDa calcium-independent phospholipase A2gamma (iPLA2gamma) isoform in Sf9 cells, purification of the N-terminal His-tagged enzyme by affinity chromatography, and the identification of its remarkable substrate selectivity that results in the highly selective generation of 2-arachidonoyl lysophosphatidylcholine. Mass spectrometric analyses demonstrated that purified iPLA2gamma hydrolyzed saturated or monounsaturated aliphatic groups readily from either the sn-1 or sn-2 positions of phospholipids. In addition, purified iPLA2gamma effectively liberated arachidonic acid from the sn-2 position of plasmenylcholine substrates. In contrast, incubation of iPLA2gamma with 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine resulted in the rapid release of palmitic acid and the selective accumulation of 2-arachidonoyl lysophosphatidylcholine (LPC), which was not metabolized further by iPLA2gamma. The putative regiospecificity of the 2-arachidonoyl LPC product was authenticated by its diagnostic fragmentation pattern during tandem mass spectrometric analysis. To identify the physiological relevance of iPLA2gamma-mediated 2-arachidonoyl LPC production utilizing naturally occurring membranes, we incubated purified rat hepatic peroxisomes with iPLA2gamma and similarly identified the selective accumulation of 2-arachidonoyl LPC. Furthermore, tandem mass spectrometric analysis demonstrated that 2-arachidonoyl LPC is a natural product in human myocardium, a tissue in which iPLA2gamma expression is robust. Because 2-arachidonoyl LPC represents a key branch point intermediate that can potentially lead to a variety of bioactive molecules in eicosanoid signaling (e.g. arachidonic acid, 2-arachidonoylglycerol), these results have uncovered a novel eicosanoid selective pathway through iPLA2gamma-mediated 2-arachidonoyl LPC production to amplify and diversify the repertoire of biologic lipid second messengers in response to cellular stimulation.

Shotgun lipidomics: electrospray ionization mass spectrometric analysis and quantitation of cellular lipidomes directly from crude extracts of biological samples

Lipidomics, after genomics and proteomics, is a newly and rapidly expanding research field that studies cellular lipidomes and the organizational hierarchy of lipid and protein constituents mediating life processes. Lipidomics is greatly facilitated by recent advances in, and novel applications of, electrospray ionization mass spectrometry (ESI/MS). In this review, we will focus on the advances in ESI/MS, which have facilitated the development of shotgun lipidomics and the utility of intrasource separation as an enabling strategy for utilization of 2D mass spectrometry in shotgun lipidomics of biological samples. The principles and experimental details of the intrasource separation approach will be extensively discussed. Other ESI/MS approaches towards the quantitative analyses of global cellular lipidomes directly from crude lipid extracts of biological samples will also be reviewed and compared. Multiple examples of lipidomic analyses from crude lipid extracts employing these approaches will be given to show the power of ESI/MS techniques in lipidomics. Currently, modern society is plagued by the sequelae of lipid-related diseases. It is our hope that the integration of these advances in multiple disciplines will catalyze the development of lipidomics, and such development will lead to improvements in diagnostics and therapeutics, which will ultimately result in the extended longevity and an improved quality of life for humankind.

Hot topics in phospholipase A2 field

As detailed in previous reviews, phospholipase A2 (PLA2) enzymes belonging to the secretory PLA2 (sPLA2), cytosolic PLA2 (cPLA2), and Ca2+-independent PLA2 (iPLA2) families may play specific physiologic and pathologic roles. In the past two years, there have been considerable advances in the understanding of the regulatory functions of individual PLA2s. This short article focuses on the latest topics in the PLA2 field, which have offered new insights into this intriguing enzyme family. Specifically, I describe a novel cellular action and unexplored in vivo functions of sPLA2, expanding regulatory aspects of cPLA2, and unique functional roles of iPLA2 in apoptosis, Ca2+ homeostasis, and myocardial ischemia.

Group VIB Ca2+-independent phospholipase A2gamma promotes cellular membrane hydrolysis and prostaglandin production in a manner distinct from other intracellular phospholipases A2

Although group VIA Ca2+-independent phospholipase A2beta (iPLA2beta) has been implicated in various cellular events, the functions of other iPLA2 isozymes remain largely elusive. In this study, we examined the cellular functions of group VIB iPLA2gamma. Lentiviral transfection of iPLA2gamma into HEK293 cells resulted in marked increases in spontaneous, stimulus-coupled, and cell death-associated release of arachidonic acid (AA), which was converted to prostaglandin E2 with preferred cyclooxygenase (COX)-1 coupling. Conversely, treatment of HEK293 cells with iPLA2gamma small interfering RNA significantly reduced AA release, indicating the participation of endogenous iPLA2gamma. iPLA2gamma protein appeared in multiple sizes according to cell types, and a 63-kDa form was localized mainly in peroxisomes. Electrospray ionization mass spectrometry of cellular phospholipids revealed that iPLA2gamma and other intracellular PLA2 enzymes acted on different phospholipid subclasses. Transfection of iPLA2gamma into HCA-7 cells also led to increased AA release and prostaglandin E2 synthesis via both COX-1 and COX-2, with a concomitant increase in cell growth. Immunohistochemistry of human colorectal cancer tissues showed elevated expression of iPLA2gamma in adenocarcinoma cells. These results collectively suggest distinct roles for iPLA2beta and iPLA2gamma in cellular homeostasis and signaling, a functional link between peroxisomal AA release and eicosanoid generation, and a potential contribution of iPLA2gamma to tumorigenesis.

Identification and distribution of endoplasmic reticulum iPLA2

Our laboratory demonstrated that endoplasmic reticulum iPLA2 (ER-iPLA2) activity protects renal cells from oxidant-induced cell death and lipid peroxidation. The goals of this study were to determine the PLA2 isoform(s) responsible for ER-iPLA2 activity in different species and tissues. ER-iPLA2 activity was observed in microsomes from rabbit and rat kidney, heart, and brain as well as in human kidney (Caki-1 and HEK293) and glioblastoma (A172) cell lines. Reverse transcriptase-polymerase chain reaction results demonstrated the presence of iPLA2gamma (group VIB PLA2) message in all tissues tested. Immunoblot analysis and PLA2 inhibitor studies with methyl arachidonyl fluorophosphonate and enantiomers of bromoenol lactone demonstrated that the ER-iPLA2 in rabbit kidney and heart and rat kidney is iPLA2gamma. These results demonstrate the expression of ER-iPLA2gamma (group VIB) across species and tissues, and suggest that iPLA2gamma may play critical roles in oxidant-induced cell injury.

Regulation of arachidonic acid availability for eicosanoid production

Mammalian cells have developed specific pathways for the incorporation, remodeling, and release of arachidonic acid. Acyltransferase and transacylase pathways function to regulate the levels of esterified arachidonic acid in specific phospholipid pools. There are several distinct, differentially regulated phospholipases A2in cells that mediate agonist-induced release of arachidonic acid. These pathways are important in controlling cellular levels of free arachidonic acid. Both arachidonic acid and its oxygenated metabolites are potent bioactive mediators that regulate a myriad of physiological and pathophysiological processes.Key words: phospholipase A2, arachidonic acid, eicosanoid, phospholipid.