Nuclear location of PLA2-I in proliferative cells

Group V Secretory Phospholipase A2 Regulates Endocytosis of Acetylated LDL by Transcriptional Activation of PGK1 in RAW264.7 Macrophage Cell Line

AIMS

It was suggested that group V secretory phospholipase A₂ (sPLA2-V) existed in the nucleus. This study examined whether nuclear sPLA₂-V plays a role in endocytosis of acetylated low-density lipoprotein (AcLDL) in monocyte/macrophage-like cell line RAW264.7 cells.

METHODS

RAW264.7 cells were transfected with shRNA vector targeting sPLA₂-V (sPLA₂-V-knockdown [KD] cells) or empty vector (sPLA₂-V-wild-type [WT] cells). AcLDL endocytosis was assessed by incubation with ¹²⁵I-AcLDL or AcLDL conjugated with pHrodo. Actin polymerization was assessed by flow cytometry using Alexa Fluor 546-phalloidin.

RESULTS

In immunofluorescence microscopic studies, sPLA₂-V was detected in the nucleus. ChIP-Seq and ChIP-qPCR analyses showed binding of sPLA₂-V to the promoter region of the phosphoglycerate kinase 1 (Pgk1) gene. In the promoter assay, sPLA₂-V-KD cells had lower promoter activity of the Pgk1 gene than sPLA₂-V-WT cells, and this decrease could be reversed by transfection with a vector encoding sPLA₂-V-H48Q that lacks enzymatic activity. Compared with sPLA₂-V-WT cells, sPLA₂-V-KD cells had decreased PGK1 protein expression, beclin 1 (Beclin1) phosphorylation at S30, and class III PI3-kinase activity that could also be restored by transfection with sPLA₂-V-H48Q. sPLA₂-V-KD cells had impaired actin polymerization and endocytosis, which was reversed by introduction of sPLA₂-V-H48Q or PGK1 overexpression. In sPLA₂-V-WT cells, siRNA-mediated depletion of PGK1 suppressed Beclin1 phosphorylation and impaired actin polymerization and intracellular trafficking of pHrodo-conjugated AcLDL.

CONCLUSIONS

Nuclear sPLA₂-V binds to the Pgk1 gene promoter region and increases its transcriptional activity. sPLA₂-V regulates AcLDL endocytosis through PGK1-Beclin1 in a manner that is independent of its enzymatic activity in RAW264.7 cells.

The neurotoxic secreted phospholipase A2 from the Vipera a. ammodytes venom targets cytochrome c oxidase in neuronal mitochondria

The β-neurotoxic secreted phospholipases A2 (sPLA2s) block neuro-muscular transmission by poisoning nerve terminals. Damage inflicted by such sPLA2s (β-ntx) on neuronal mitochondria is characteristic, very similar to that induced by structurally homologous endogenous group IIA sPLA2 when its activity is elevated, as, for example, in the early phase of Alzheimer's disease. Using ammodytoxin (Atx), the β-ntx from the venom of the nose-horned viper (Vipera a. ammodytes), the sPLA2 receptor R25 has been detected in neuronal mitochondria. This receptor has been purified from porcine cerebral cortex mitochondria by a new Atx-affinity-based chromatographic procedure. Mass spectrometry analysis revealed R25 to be the subunit II of cytochrome c oxidase (CCOX), an essential constituent of the respiratory chain complex. CCOX was confirmed as being the first intracellular membrane receptor for sPLA2 by alternative Atx-affinity-labellings of purified CCOX, supported also by the encounter of Atx and CCOX in PC12 cells. This discovery suggests the explanation of the mechanism by which β-ntx hinders production of ATP in poisoned nerve endings. It also provides a new insight into the potential function and dysfunction of endogenous GIIA sPLA2 in mitochondria.

Nuclear Lipid Signaling: Novel Role of Eicosanoids

Nuclear lipid signaling is an established, widespread mechanism that operates in multiple cellular processes including proliferative and differentiative responses to a variety of stimuli. In this literature review with key references highlighted, we put forward the hypothesis that differential flow through various intracrine mechanisms can dictate resultant cellular actions such as mitosis, differentiation, or apoptosis.

On the role of protein disulfide isomerase in the retrograde cell transport of secreted phospholipases A2

Following the finding that ammodytoxin (Atx), a neurotoxic secreted phospholipase A₂ (sPLA₂) in snake venom, binds specifically to protein disulfide isomerase (PDI) in vitro we show that these proteins also interact in living rat PC12 cells that are able to internalize this group IIA (GIIA) sPLA₂. Atx and PDI co-localize in both differentiated and non-differentiated PC12 cells, as shown by fluorescence microscopy. Based on a model of the complex between Atx and yeast PDI (yPDI), a three-dimensional model of the complex between Atx and human PDI (hPDI) was constructed. The Atx binding site on hPDI is situated between domains b and b’. Atx interacts hPDI with an extensive area on its interfacial binding surface. The mammalian GIB, GIIA, GV and GX sPLA₂s have the same fold as Atx. The first three sPLA₂s have been detected intracellularly but not the last one. The models of their complexes with hPDI were constructed by replacement of Atx with the respective mammalian sPLA₂ in the Atx—hPDI complex and molecular docking of the structures. According to the generated models, mammalian GIB, GIIA and GV sPLA₂s form complexes with hPDI very similar to that with Atx. The contact area between GX sPLA₂ and hPDI is however different from that of the other sPLA₂s. Heterologous competition of Atx binding to hPDI with GV and GX sPLA₂s confirmed the model-based expectation that GV sPLA₂ was a more effective inhibitor than GX sPLA₂, thus validating our model. The results suggest a role of hPDI in the (patho)physiology of some snake venom and mammalian sPLA₂s by assisting the retrograde transport of these molecules from the cell surface. The sPLA₂–hPDI model constitutes a valuable tool to facilitate further insights into this process and into the (patho)physiology of sPLA₂s in relation to their action intracellularly.

PLA2R1: expression and function in cancer

The phospholipase A2 receptor 1 (PLA2R1 or PLA2R) was isolated twenty years ago for its ability to bind several secretory phospholipase A2 proteins (sPLA2). Since its identification, it has attracted only a limited interest, mainly in the sPLA2 biology field, as it is viewed uniquely as a regulator of sPLA2 activities. Recent discoveries outline novel important functions of this gene in cancer biology. Indeed, PLA2R1 gain or loss of function experiments in vitro and in vivo shows that this receptor promotes several tumor suppressive responses including senescence, apoptosis and inhibition of transformation. Supporting a tumor suppressive role of PLA2R1, its expression decreases in numerous cancers, and known oncogenes such as HIF2α and c-MYC repress its expression. PLA2R1 promoter methylation, a classical way to repress tumor suppressive gene expression in cancer cells, is observed in leukemia, in kidney and in breast cancer cells. Mechanistically, PLA2R1 activates the kinase JAK2 and orients its activity towards a tumor suppressive one. PLA2R1 also promotes accumulation of reactive oxygen species which induce cell death and senescence. This review compiles recent data demonstrating an unexpected tumor suppressive role of PLA2R1 and outlines the future work needed to improve our knowledge of the functions of this gene in cancer.

Phospholipase A(2) activation by poultry particulate matter is mediated through extracellular signal-regulated kinase in lung epithelial cells: regulation of interleukin-8 release

The mechanisms of poultry particulate matter (PM)-induced agricultural respiratory disorders are not thoroughly understood. Hence, it is hypothesized in this article that poultry PM induces the release of interleukin-8 (IL-8) by lung epithelial cells that is regulated upstream by the concerted action of cytosolic phospholipase A2 (cPLA2) and extracellular signal-regulated kinase (ERK). To test this hypothesis, the widely used cultured human lung epithelial cells (A549) were chosen as the model system. Poultry PM caused a significant activation of PLA2 in A549 cells, which was attenuated by AACOCF3 (cPLA2 inhibitor) and PD98059 (ERK-1/2 upstream inhibitor). Poultry PM induced upstream ERK-1/2 phosphorylation and downstream cPLA2 serine phosphorylation, in a concerted fashion, in cells with enhanced association of ERK-1/2 and cPLA2. The poultry PM-induced cPLA2 serine phosphorylation and IL-8 release were attenuated by AACOCF3, PD98059, and by transfection with dominant-negative ERK-1/2 DNA in cells. The poultry PM-induced IL-8 release by the bone marrow-derived macrophages of cPLA2 knockout mice was significantly lower. For the first time, this study demonstrated that the poultry PM-induced IL-8 secretion by human lung epithelial cells was regulated by cPLA2 activation through ERK-mediated serine phosphorylation, suggesting a mechanism of airway inflammation among poultry farm workers.

cPLA2α knockout mice exhibit abnormalities in the architecture and synapses of cortical neurons

Cytosolic phospholipase A2α (cPLA2α) affects membrane fluidity and permeability by catalyzing the hydrolysis of membrane phospholipids. We hypothesize that cPLA2α deficiency induces rigidity and architectural changes in cell membranes, especially in large cortical neurons. These membrane changes are discernible using light and electron microscopy. Through careful comparison with wild-type counterparts, we observed significant morphological changes in cortical neurons of cPLA2α knockout mice. These changes included the following: (1) increased numbers of nucleoli and enlarged nuclei, (2) narrower spaces between the inner and outer nuclear membranes, (3) reduced numbers of nuclear pores and altered nuclear pore structure, and (4) morphological changes in synaptic clefts. These results further suggest that cPLA2α and its cleaved arachidonic acids play important roles in cortical neuronal maturation and in normal neurochemical processes.

Effect of trifluoperazine on toxicity, HIF-1α induction and hepatocyte regeneration in acetaminophen toxicity in mice

Oxidative stress and mitochondrial permeability transition (MPT) are important mechanisms in acetaminophen (APAP) toxicity. The MPT inhibitor trifluoperazine (TFP) reduced MPT, oxidative stress, and toxicity in freshly isolated hepatocytes treated with APAP. Since hypoxia inducible factor-one alpha (HIF-1α) is induced very early in APAP toxicity, a role for oxidative stress in the induction has been postulated. In the present study, the effect of TFP on toxicity and HIF-1α induction in B6C3F1 male mice treated with APAP was examined. Mice received TFP (10mg/kg, oral gavage) prior to APAP (200mg/kg IP) and at 7 and 36h after APAP. Measures of metabolism (hepatic glutathione and APAP protein adducts) were comparable in the two groups of mice. Toxicity was decreased in the APAP/TFP mice at 2, 4, and 8h, compared to the APAP mice. At 24 and 48h, there were no significant differences in toxicity between the two groups. TFP lowered HIF-1α induction but also reduced the expression of proliferating cell nuclear antigen, a marker of hepatocyte regeneration. TFP can also inhibit phospholipase A(2), and cytosolic and secretory PLA(2) activity levels were reduced in the APAP/TFP mice compared to the APAP mice. TFP also lowered prostaglandin E(2) expression, a known mechanism of cytoprotection. In summary, the MPT inhibitor TFP delayed the onset of toxicity and lowered HIF-1α induction in APAP treated mice. TFP also reduced PGE(2) expression and hepatocyte regeneration, likely through a mechanism involving PLA(2).

G protein-coupled receptor signalling in the cardiac nuclear membrane: evidence and possible roles in physiological and pathophysiological function

G protein-coupled receptors (GPCRs) play key physiological roles in numerous tissues, including the heart, and their dysfunction influences a wide range of cardiovascular diseases. Recently, the notion of nuclear localization and action of GPCRs has become more widely accepted. Nuclear-localized receptors may regulate distinct signalling pathways, suggesting that the biological responses mediated by GPCRs are not solely initiated at the cell surface but may result from the integration of extracellular and intracellular signalling pathways. Many of the observed nuclear effects are not prevented by classical inhibitors that exclusively target cell surface receptors, presumably because of their structures, lipophilic properties, or affinity for nuclear receptors. In this topical review, we discuss specifically how angiotensin-II, endothelin, β-adrenergic and opioid receptors located on the nuclear envelope activate signalling pathways, which convert intracrine stimuli into acute responses such as generation of second messengers and direct genomic effects, and thereby participate in the development of cardiovascular disorders.

Identification of a 42-kDa Group IV cPLA2-activating protein, cPLAPγ, as a GTP-binding protein in the bovine brain

Brain tissue contains multiple forms of Phospholipase A(2) (PLA(2)) whose activities are involved in intracellular and intercellular signalling related to normal functions such as long-term potentiation, neurotransmitter release, cell growth and differentiation. Among them, we focused on regulatory mechanism of cPLA(2)α (Group IVA cytosolic PLA(2)) in brain tissue. In the present study, we report the identification of a cPLA(2)-activating protein (cPLAP) in the bovine brain. cPLAP activity appeared as two major peaks with molecular masses of 200 and 42 kDa in a Superose 12 gel filtration FPLC column. The 42-kDa form of cPLAP, designated cPLAPγ, was further purified using a Mono S FPLC column to near homogeneity and characterized to as a GTP-binding protein (G protein). Metabolic labelling and immunoprecipitation studies revealed that cPLAPγ associates with cPLA(2) in vitro and co-immunoprecipitates with [(35)S]-cPLA(2). Notably, cPLAPγ rendered cPLA(2) fully activated at submicromolar concentrations of Ca(2+). These results suggest that cPLAPγ may act as a G protein, activating cPLA(2)α prior to reaching full intracellular Ca(2+) concentrations.

Emerging roles of secreted phospholipase A2 enzymes: Lessons from transgenic and knockout mice

Among the emerging phospholipase A(2) (PLA(2)) superfamily, the secreted PLA(2) (sPLA(2)) family consists of low-molecular-mass, Ca(2+)-requiring extracellular enzymes with a His-Asp catalytic dyad. To date, more than 10 sPLA(2) enzymes have been identified in mammals. Individual sPLA(2)s exhibit unique tissue and cellular localizations and enzymatic properties, suggesting their distinct pathophysiological roles. Despite numerous enzymatic and cell biological studies on this enzyme family in the past two decades, their precise in vivo functions still remain largely obscure. Recent studies using transgenic and knockout mice for several sPLA(2) enzymes, in combination with lipidomics approaches, have opened new insights into their distinct contributions to various biological events such as food digestion, host defense, inflammation, asthma and atherosclerosis. In this article, we overview the latest understanding of the pathophysiological functions of individual sPLA(2) isoforms fueled by studies employing transgenic and knockout mice for several sPLA(2)s.

Structural basis of the significant calmodulin-induced increase in the enzymatic activity of secreted phospholipases A(2)

Ammodytoxin (Atx), a neurotoxic secreted phospholipase A(2) (sPLA(2)), forms a high-affinity complex with calmodulin (CaM). The latter substantially increases the enzymatic activity of Atx under both non-reducing and reducing conditions, and the activity enhancement was accompanied, but not caused, by conformational stabilization of the enzyme. In this work, the energetically most favorable model of the complex was generated, making use of interaction site mapping, mutagenesis data and protein-docking algorithms. The model explains, in structural terms, the observed effects of stabilization and activity enhancement of the neurotoxic sPLA(2) by CaM. The structures of four mammalian sPLA(2) isoforms, groups IB, IIA, V and X, having the same fold as Atx, were superimposed on the structure of Atx in the complex with CaM. According to the generated models, the group V and X sPLA(2)s, but not the group IB and IIA enzymes, form stable complexes with CaM, which should also result in the augmentation of their enzymatic activity. By confirming the latter, the presented model is validated as a valuable tool to investigate the as yet unexplained role of CaM in the pathophysiology of snake venom and mammalian sPLA(2)s.

Recombinant production and properties of binding of the full set of mouse secreted phospholipases A2 to the mouse M-type receptor

To date, 12 secreted phospholipases A2 (sPLA2s) have been identified in the mouse species and divided into three structural collections (I/II/V/X, III, and XII). On the basis of their different molecular properties and tissue distributions, each sPLA2 is likely to exert distinct functions by acting as an enzyme or ligand for specific soluble proteins or receptors, among which the M-type receptor is the best-characterized target. Here, we present the properties of binding of the full set of mouse sPLA2s to the mouse M-type receptor. All enzymes have been produced in Escherichia coli or insect cells, and their properties of binding to the cloned and native M-type receptor have been determined. sPLA2s IB, IIA, IIE, IIF, and X are high-affinity ligands (K0.5 = 0.3-3 nM); sPLA2s IIC and V are low-affinity ligands (K0.5 = 30-75 nM), and sPLA2s IID, III, XIIA, and XIIB bind only very weakly or do not bind to the M-type receptor (K0.5 > 100 nM). Three exogenous parvoviral group XIII PLA2s and two fungal group XIV sPLA2s do not bind to the receptor. Together, these results indicate that the mouse M-type receptor is selective for only a subset of mouse sPLA2s from the group I/II/V/X structural collection. Binding of mouse sPLA2s to a recombinant soluble mouse M-type receptor leads in all cases to inhibition of enzymatic activity, and the extent of deglycosylation of the receptor decreases yet does not abolish sPLA2 binding. The physiological meaning of binding of sPLA2 to the M-type receptor is discussed on the basis of our current knowledge of sPLA2 functions.

Nuclear Ptdlns(3,4,5)P3 signaling: an ongoing story

Phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3,4,5)P(3)) is linked to a variety of cellular functions, such as growth, cell survival, and differentiation. Ptdlns(3,4,5)P(3) is primarily synthesized by class I phosphoinositide 3-kinases and its hydrolysis by two 3-phosphoinositide 3-phosphatases, PTEN and SHIP proteins, leads to the production of two other second messengers, Ptdlns(4,5)P(2) and Ptdlns(3,4)P(2), respectively. Evidence accumulated over the last years strongly suggest that Ptdlns(3,4,5)P(3) is an important component of signaling pathway operating within the nucleus. Moreover, recent advances indicated that nuclear translocation of cell surface receptors could activate nuclear phosphoinositide 3-kinase suggesting a new mode of signal transduction. The aim of this review is intended to summarize the state of our knowledge on nuclear Ptdlns(3,4,5)P(3) and its metabolizing enzymes, and to highlight the emerging roles for intranuclear Ptdlns(3,4,5)P(3).

Completing the cycles; the dynamics of endonuclear lipidomics

Signal transductions via periodic generation and mobilisation of lipid second messengers within the nuclear matrix of eukaryotic cells have focused renewed attention on their precursor phospholipids' location, structure, form and function. The nuclear matrix contains and supports dynamic pools of phosphatidylcholine and phosphatidylinositol which serve as parent molecules of lipid second messengers but also of other phospholipids requiring cyclical replacement as cells proliferate. Applications of new, highly sensitive and specific analytical methodologies based on tandem electrospray ionisation mass spectrometry and the use of stable isotopes have allowed both static and dynamic lipidomic profiling of these endonuclear phospholipid pools. Together with more conventional enzymatic analyses and evaluation of the effect of specific "knock-out" of phospholipid transfer capacity, a number of important principles have been established. Specifically, a compartmental capacity to synthesise and remodel highly saturated phosphatidylcholine exists alongside transport mechanisms that facilitate the nuclear import of phosphatidylinositol and other phospholipids synthesised elsewhere within the cell. Subnuclear fractionation and the use of newly emerging techniques for sensitive lipidomic profiling of polyphosphoinositides, diacylglycerols and phosphatidate molecular species offer the potential for further significant advances in the near future.

Hydrolysis of nuclear phospholipids in relation with proliferative state in uterine stromal cells

The current study examined the metabolism of phospholipid (PL) in the whole cell homogenate and in the nuclear fraction in proliferative and non-proliferative uterine stromal cells (U(III) cells). Growth arrested cells were obtained either from contact-inhibited confluent cells or from proliferative cells treated with aristolochic acid (AR) for 2 days. Fatty acid composition and fatty acid amount of both total and nuclear PL were not significantly different between proliferative, confluent and AR-treated cells. In contrast, marked differences were observed in the incorporation of [(3)H]AA, with greater incorporation in proliferative cells than in confluent or AR-treated cells, particularly in nuclear PL. Considering endogenous level of arachidonic acid (AA) in total and nuclear PL, we found that AA turnover in nuclear PL was especially high compared to that in total PL and that this difference was accentuated in proliferative cells compared to non-proliferative cells. Interestingly, [(3)H]AA incorporation and AA turnover in proliferative, confluent and AR-treated cells vary accordingly to the expression, activity and/or content of pancreatic phospholipase A(2) (PLA(2)-I) in the nuclear compartment of these cells that we reported in previous studies. The changes in metabolism of nuclear PL during cell proliferation are consistent with an enhanced PL hydrolysis that could involve PLA(2)-I.

Nuclear prostaglandin receptors: role in pregnancy and parturition?

The key regulatory role of prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] in the maintenance of pregnancy and initiation of parturition has been established. However, our understanding of how these events are fine-tuned by the recruitment of specific signaling pathways remains unclear. Whereas, initial thoughts were that PGs were lipophilic and would easily cross cell membranes without specific receptors or transport processes, it has since been realized that PG signaling occurs via specific cell surface G-protein coupled receptors (GPCRs) coupled to classical adenylate cyclase or inositol phosphate signaling pathways. Furthermore, specific PG transporters have been identified and cloned adding a further level of complexity to the regulation of paracrine action of these potent bioactive molecules. It is now apparent that PGs also activate nuclear receptors, opening the possibility of novel intracrine signaling mechanisms. The existence of intracrine signaling pathways is further supported by accumulating evidence linking the perinuclear localization of PG synthesizing enzymes with intracellular PG synthesis. This review will focus on the evidence for a role of nuclear actions of PGs in the regulation of pregnancy and parturition.

R25 is an intracellular membrane receptor for a snake venom secretory phospholipase A(2)

Ammodytoxin is a presynaptically neurotoxic (beta-neurotoxic) snake venom secretory phospholipase A(2) (sPLA(2)). We detected a 25 kDa protein which binds the toxin with very high affinity (R25) in porcine cerebral cortex. Here we show that R25 is an integral membrane protein with intracellular localisation. It is the first sPLA(2) receptor known to date that localises to intracellular membranes. Centrifugation on sucrose gradients was used to fractionate porcine cerebral cortex. The subcellular composition of the fractions was determined by following the distribution of organelle-specific markers. The distribution of R25 in the fractions matched the distribution of the mitochondrial marker succinate dehydrogenase, but not the markers for plasma membrane, lysosomes, endoplasmic reticulum, synaptic and secretory vesicles. R25 most likely resides in mitochondria, which are known to be targets for sPLA(2) neurotoxins in the nerve ending and are potentially implicated in the process of beta-neurotoxicity.

Modulation of pro-inflammatory gene expression by nuclear lysophosphatidic acid receptor type-1

Lysophosphatidic acid (LPA) is a bioactive molecule involved in inflammation, immunity, wound healing, and neoplasia. Its pleiotropic actions arise presumably by interaction with their cell surface G protein-coupled receptors. Herein, the presence of the specific nuclear lysophosphatidic acid receptor-1 (LPA1R) was revealed in unstimulated porcine cerebral microvascular endothelial cells (pCMVECs), LPA1R stably transfected HTC4 rat hepatoma cells, and rat liver tissue using complementary approaches, including radioligand binding experiments, electron- and cryomicroscopy, cell fractionation, and immunoblotting with three distinct antibodies. Coimmunoprecipitation studies in enriched plasmalemmal fractions of unstimulated pCMVEC showed that LPA1Rs are dually sequestrated in caveolin-1 and clathrin subcompartments, whereas in nuclear fractions LPA1R appeared primarily in caveolae. Immunofluorescent assays using a cell-free isolated nuclear system confirmed LPA1R and caveolin-1 co-localization. In pCMVEC, LPA-stimulated increases in cyclooxygenase-2 and inducible nitric-oxide synthase RNA and protein expression were insensitive to caveolea-disrupting agents but sensitive to LPA-generating phospholipase A2 enzyme and tyrosine kinase inhibitors. Moreover, LPA-induced increases in Ca2+ transients and/or iNOS expression in highly purified rat liver nuclei were prevented by pertussis toxin, phosphoinositide 3-kinase/Akt inhibitor wortmannin and Ca2+ chelator and channel blockers EGTA and SK&F96365, respectively. This study describes for the first time the nucleus as a potential organelle for LPA intracrine signaling in the regulation of pro-inflammatory gene expression.

Potentiation of tumor necrosis factor alpha-induced secreted phospholipase A2 (sPLA2)-IIA expression in mesangial cells by an autocrine loop involving sPLA2 and peroxisome proliferator-activated receptor alpha activation

In rat mesangial cells, exogenously added secreted phospholipases A2 (sPLA2s) potentiate the expression of pro-inflammatory sPLA2-IIA first induced by cytokines like tumor necrosis factor-alpha (TNFalpha) and interleukin-1 beta. The transcriptional pathway mediating this effect is, however, unknown. Because products of PLA2 activity are endogenous activators of peroxisome proliferator-activated receptor alpha (PPAR alpha, we postulated that sPLA2s mediate their effects on sPLA2-IIA expression via sPLA2 activity and subsequent PPAR alpha activation. This study shows that various sPLA2s, including venom enzymes, human sPLA2-IIA, and wild-type and catalytically inactive H48Q mutant of porcine pancreatic sPLA2-IB, enhance the TNF alpha-induced sPLA2-IIA expression at the mRNA and protein levels. In cells transfected with luciferase sPLA2-IIA promoter constructs, sPLA2s are active only when the promoter contains a functional PPRE-1 site. The effect of exogenous sPLA2s is also blocked by the PPAR alpha inhibitor MK886. Interestingly, the expression of sPLA2-IIA induced by TNF alpha alone is also attenuated by MK886, by the sPLA2-IIA inhibitor LY311727, by heparinase, which prevents the binding of sPLA2-IIA to heparan sulfate proteoglycans, and by the specific cPLA2-alpha inhibitor pyrrolidine-1. Together, these data indicate that sPLA2-IIA released from mesangial cells by TNF alpha stimulates its own expression via an autocrine loop involving cPLA2 and PPAR alpha. This signaling pathway is also used by exogenously added sPLA2s including pancreatic sPLA2-IB and is distinct from that used by TNF alpha.

Intracellular renin and the nature of intracrine enzymes

Recently, the binding of renin and prorenin to cellular receptors with the subsequent generation of second messengers and the production of physiological effects has been demonstrated. In addition, the internalization of prorenin by target cells has been associated with increased cellular synthesis of angiotensin and cardiac pathology. Also, a renin transcript lacking the sequences encoding a secretory signal has been reported, and this transcript appears to produce a renin that acts in the cell that synthesized it. Some years ago, we coined the term intracrine for a peptide hormone or factor that acts in the intracellular space either after internalization or retention in its cell of synthesis. Thus defined, a wide variety of peptides display intracrine functionality, including hormones, growth factors, transcription factors, and enzymes. For example, considerable evidence indicates that angiotensin II is an intracrine. Also, general principles of intracrine functionality have been developed. Thus, recent evidence demonstrates that the prorenin/renin molecule is an intracrine enzyme. Here, the actions of intracrine enzymes (angiogenin, phosphoglucose isomerase, phospholipase A2, granzyme A and B, thioredoxin, platelet-derived endothelial growth factor, and serine protease inhibitors) are reviewed. The relation of prorenin/renin to other intracrine enzymes, and to intracrines in general, is discussed.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Nuclear lipid signaling

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.

Phospholipase A2 receptor: a regulator of biological functions of secretory phospholipase A2

The phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family that includes the mannose receptor. PLA2R regulates a variety of biological responses elicited by specific types of secretory PLA2s (sPLA2s). Group IB sPLA2 (sPLA2-IB) acts as an endogenous PLA2R ligand to induce cell proliferation, cell migration, and lipid mediator production. Analysis of PLA2R-deficient mice has suggested a potential role of the sPLA2-IB/PLA2R pathway in the production of pro-inflammatory cytokines in endotoxic shock. PLA2R is also involved in the clearance of sPLA2s, including group X sPLA2 (sPLA2-X) and a particular type of snake venom sPLA2, and clearance suppresses their potent enzymatic activities. In the circulation, the soluble form of PLA2R is constitutively present as anendogenous inhibitor of sPLA2s. This review will focus on recent findings on the roles of PLA2R in regulating sPLA2 functions and summarize what is known about the otherbinding proteins for mammalian and snake venom sPLA2s.

Implication of three isoforms of PLA(2) in human T-cell proliferation

We observed that human (Jurkat) T-cells constitutively expressed the mRNA, encoding for the four isoforms of phospholipase A(2) (PLA(2)), i.e. two secretory (type IB and type V), and two cytosolic (type IV, Ca(2+)-dependent and type VI, Ca(2+)-independent). In order to assess whether these PLA(2) isoforms are active, we labeled Jurkat T-cells with [(3)H]arachidonic acid ([(3)H]AA) and determined its release into the extracellular medium in the presence of phorbol 12-myristate 13-acetate (PMA) and ionomycin. The three PLA(2) isoforms seem functional as aristolochic acid and bromoenol lactone (BEL), the respective inhibitors of type IB/type V and type VI PLA(2)s, significantly inhibited the release of free [(3)H]AA. On the other hand, arachidonyl trifluoromethyl ketone (AACOCF(3)), an inhibitor of type IV PLA(2), failed to curtail significantly the release of free [(3)H]AA into the extracellular medium. We assessed the implication of these PLA(2) isoforms in transcription of the interleukin-2 (IL-2) gene, involved in T-cell proliferation. Hence, aristolochic acid and BEL, but not AACOCF(3), significantly inhibited the PMA and ionomycin-induced induction of mRNA of IL-2. Similarly, aristolochic acid and BEL, but not AACOCF(3), significantly inhibited the PMA and ionomycin-induced secretion of IL-2 in the culture supernatants. Together these results suggest that human Jurkat T-cells possess two secretory and two cytosolic PLA(2) isoforms and only three of them (type IB, type V and type VI) are implicated in T-cell proliferation.

The origins of intracrine hormone action

A growing number of peptide hormones and growth factors have been shown to operate in the intracellular space after either internalization or retention in their cells of synthesis. These factors, called intracrines, in many cases are expressed as multiple isoforms, traffic to nucleus or nucleolus, and regulate gene transcription. Some intracrines are angiogenic. It is here argued that intracrine action is the modern analogue of a biologically ancient mechanism for regulating message translation and ribosome synthesis. The implications of this view for research and therapeutics are discussed.

Clearance of group X secretory phospholipase A(2) via mouse phospholipase A(2) receptor

Given the potent hydrolyzing activity toward phosphatidylcholine, group X secretory phospholipase A(2) (sPLA(2)-X) elicits a marked release of arachidonic acid linked to the potent production of lipid mediators in various cell types. We have recently shown that sPLA(2)-X can also act as a ligand for mouse phospholipase A(2) receptor (PLA(2)R). Here, we found that sPLA(2)-X was internalized and degraded via binding to PLA(2)R associated with the diminished prostaglandin E(2) (PGE(2)) formation in PLA(2)R-expressing Chinese hamster ovary (CHO) cells compared to CHO cells. Indirect immunocytochemical analysis revealed that internalized sPLA(2)-X was co-localized with PLA(2)R in the punctate structures in PLA(2)R-expressing CHO cells. Moreover, in mouse osteoblastic MC3T3-E(1) cells that endogenously express the PLA(2)R, the internalized sPLA(2)-X was localized in lysosomes. These findings demonstrate that PLA(2)R acts as a clearance receptor for sPLA(2)-X to suppress its strong enzymatic activity.

Control of cell proliferation via transduction of sPLA(2)-I activity and possible PPAR activation at the nuclear level

Pancreatic phospholipase A2 (PLA(2)-I) stimulates U(III) cells proliferation, a rat uterine cell line, after binding to membrane receptors, internalization and translocation. Here, we demonstrate that during these steps of internalization, PLA(2)-I retains its hydrolytic activity and thus could exert its proliferative effect via nuclear phospholipids hydrolysis. Since fatty acids and eicosanoids released by such activity are known to be ligands of PPAR, we study the expression of these nuclear receptors and demonstrate that, in the experimental conditions where PLA(2)-I stimulates U(III) cells proliferation, PLA(2)-I also regulates PPAR expression indicating a possible mechanism of its proliferative effect.

Hydrogen peroxide activation of Ca(2+)-independent phospholipase A(2) in uterine stromal cells

In rat uterine stromal cells (U(III) cells), an oxidative stress induced by H(2)O(2) caused a dose-dependent release of arachidonic acid (AA) that was independent of intracellular Ca(2+) concentration and was not inhibited by Ca(2+)-dependent phospholipase A(2) (cPLA(2)) inhibitors, nor by protein kinase C (PKC) inhibitors or by PKC down-regulation. H(2)O(2) treatment did not impair AA esterification but significantly increased Ca(2+)-independent PLA(2) (iPLA(2)) activity. Since iPLA(2) specific inhibitor bromoenollactone almost completely suppressed the release of AA induced by H(2)O(2), we conclude that iPLA(2) activity represents the major mechanism by which H(2)O(2) increases the availability of non-esterified AA in U(III) cells. Moreover, PKC inhibitors sphingosine and calphostin C markedly potentiated the release of AA trigger by H(2)O(2), suggesting a regulatory mechanism of iPLA(2) by PKC that remains to be clarified.

Nuclear lipid signaling

There is now abundant evidence for the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope, but are actually within the nucleus, presumably not in a bilayer membrane but instead forming proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol and inositol-1,4,5-trisphosphate [Ins(1,4,5)P(3)]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet clear. Generation of diacylglycerol within the nucleus is believed to recruit protein kinase C to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P(3) may mobilize Ca(2+) from the space between the nuclear membranes and thus increase nucleoplasmic Ca(2+). Less well understood are an increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus: (i) two separate routes of synthesis of phosphatidylinositol-4,5-bisphosphate; (ii) two different sources of diacylglycerol, one being from the PI-PLC pathway, and the other probably from phosphatidylcholine; (iii) several different isoforms of PKC translocating to the nuclei; (iv) increases in activity of the PI-PLC pathway at two different points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P(3), which may have several functions, including a role in the transfer of messenger RNA (mRNA) out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and 3-phosphorylated inositol lipids.

Neuronal receptors for phospholipases A(2 )and beta-neurotoxicity

Some phospholipases A(2) interrupt neuromuscular communication by blocking the release of neurotransmitter into the synaptic cleft. Despite numerous studies, the molecular mechanism of their action is still largely obscure. In this review the best-characterized receptors for beta-neurotoxins are presented. We propose a model which could be useful in investigating the apparent inconsistency between the observed heterogeneity in the neuronal binding of beta-neurotoxins and the very similar pathomorphological and electrophysiological effects which they produce in the intoxicated tissue. We assume that beta-neurotoxins enter the nerve ending to exert their toxic effect. The model involves different pathways for phospholipase A(2) neurotoxins to reach the site of action inside the neuron, their respective extra- and intracellular neuronal receptors being key features of the pathway. Once in the nerve cell, beta-neurotoxins impair the function of the synaptic vesicles by phospholipid hydrolysis of the inner leaflet of the vesicle bilayer. The proportion of the products of the phospholipid hydrolysis, lysophospholipids and phospholipids in the membrane, has been demonstrated to be very important for the shaping of the membrane, affecting its fusogenic properties. Due to the same final step in the action of beta-neurotoxins, phospholipid hydrolysis, the consequences of their poisoning are practically identical.

Functional association of type IIA secretory phospholipase A(2) with the glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan in the cyclooxygenase-2-mediated delayed prostanoid-biosynthetic pathway

An emerging body of evidence suggests that type IIA secretory phospholipase A(2) (sPLA(2)-IIA) participates in the amplification of the stimulus-induced cyclooxygenase (COX)-2-dependent delayed prostaglandin (PG)-biosynthetic response in several cell types. However, the biological importance of the ability of sPLA(2)-IIA to bind to heparan sulfate proteoglycan (HSPG) on cell surfaces has remained controversial. Here we show that glypican, a glycosylphosphatidylinositol (GPI)-anchored HSPG, acts as a physical and functional adaptor for sPLA(2)-IIA. sPLA(2)-IIA-dependent PGE(2) generation by interleukin-1-stimulated cells was markedly attenuated by treatment of the cells with heparin, heparinase or GPI-specific phospholipase C, which solubilized the cell surface-associated sPLA(2)-IIA. Overexpression of glypican-1 increased the association of sPLA(2)-IIA with the cell membrane, and glypican-1 was coimmunoprecipitated by the antibody against sPLA(2)-IIA. Glypican-1 overexpression led to marked augmentation of sPLA(2)-IIA-mediated arachidonic acid release, PGE(2) generation, and COX-2 induction in interleukin-1-stimulated cells, particularly when the sPLA(2)-IIA expression level was suboptimal. Immunofluorescent microscopic analyses of cytokine-stimulated cells revealed that sPLA(2)-IIA was present in the caveolae, a microdomain in which GPI-anchored proteins reside, and also appeared in the perinuclear area in proximity to COX-2. We therefore propose that a GPI-anchored HSPG glypican facilitates the trafficking of sPLA(2)-IIA into particular subcellular compartments, and arachidonic acid thus released from the compartments may link efficiently to the downstream COX-2-mediated PG biosynthesis.

New developments in phospholipase A2

Some of the most recent data concerning various phospholipases A2, with special emphasis on secretory, cytosolic, and calcium-independent phospholipases A2 are summarized. Besides their contribution to the production of proinflammatory lipid mediators, the involvement of these enzymes in key cell responses such as apoptosis or tumor cell metastatic potential is also discussed, taking advantage of transgenic models based on gene invalidation by homologous recombination. The possible role of secretory and cytosolic platelet-activating factor acetyl hydrolases is also briefly mentioned. Finally, the ectopic expression in epididymis of an intestinal phospholipase B opens some novel issues as to the possible function of phospholipases in reproduction.

Phospholipid signalling in the nucleus. Een DAG uit het leven van de inositide signalering in de nucleus

Diverse methodologies, ranging from activity measurements in various nuclear subfractions to electron microscopy, have been used to demonstrate and establish that many of the key lipids and enzymes responsible for the metabolism of inositol lipids are resident in nuclei. PtdIns(4)P, PtdIns(4,5)P2 and PtdOH are all present in nuclei, as well as the corresponding enzyme activities required to synthesise and metabolise these compounds. In addition other non-inositol containing phospholipids such as phosphatidylcholine constitute a significant percentage of the total nuclear phospholipid content. We feel that it is pertinent to include this lipid in our discussion as it provides an alternative source of 1, 2-diacylglycerol (DAG) in addition to the hydrolysis of PtdIns(4, 5)P2. We discuss at length data related to the sources and possible consequences of nuclear DAG production as this lipid appears to be increasingly central to a number of general physiological functions. Data relating to the existence of alternative pathways of inositol phospholipid synthesis, the role of 3-phosphorylated inositol lipids and lipid compartmentalisation and transport are reviewed. The field has also expanded to a point where we can now also begin to address what role these lipids play in cellular proliferation and differentiation and hopefully provide avenues for further research.