A novel phospholipase A2 associated with nuclear matrix: stimulation of the activity and modulation of the Ca2+ dependency by polyphosphoinositides

Review of four major distinct types of human phospholipase A2

The phospholipase A2 superfamily of enzymes plays a significant role in the development and progression of numerous inflammatory diseases. Through their catalytic action on membrane phospholipids, phospholipases are the upstream regulators of the eicosanoid pathway releasing free fatty acids for cyclooxygenases, lipoxygenases, and cytochrome P450 enzymes which produce various well-known inflammatory mediators including leukotrienes, thromboxanes and prostaglandins. Elucidating the association of phospholipases A2 with the membrane, the extraction and binding of phospholipid substrates, and their interactions with small-molecule inhibitors is crucial for the development of new anti-inflammatory therapeutics. Studying phospholipases has been challenging because they act on the surface of cellular membranes and micelles. Multidisciplinary approaches including hydrogen/deuterium exchange mass spectrometry, molecular dynamics simulations, and other computer-aided drug design techniques have been successfully employed by our laboratory to study interactions of phospholipases with membranes, phospholipid substrates and inhibitors. This review summarizes the application of these techniques to study four human recombinant phospholipases A2.

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.

Age-related changes in retinoic, docosahexaenoic and arachidonic acid modulation in nuclear lipid metabolism

The aim of this work was to study how age-related changes could modify several enzymatic activities that regulate lipid mediator levels in nuclei from rat cerebellum and how these changes are modulated by all-trans retinoic acid (RA), docosahexaenoic acid (DHA) and arachidonic acid (AA). The higher phosphatidate phosphohydrolase activity and lower diacylglycerol lipase (DAGL) activity observed in aged animals compared with adults could augment diacylglycerol (DAG) availability in the former. Additionally, monoacylglycerol (MAG) availability could be high due to an increase in lysophosphatidate phosphohydrolase (LPAPase) activity and a decrease in monocylglycerol lipase activity. Interestingly, RA, DHA and AA were observed to modulate these enzymatic activities and this modulation was found to change in aged rats. In adult nuclei, whereas RA led to high DAG and MAG production through inhibition of their hydrolytic enzymes, DHA and AA promoted high MAG production by LPAPase and DAGL stimulation. In contrast, in aged nuclei RA caused high MAG generation whereas DHA and AA diminished it through LPAPase activity modulation. These results demonstrate that aging promotes a different nuclear lipid metabolism as well as a different type of non-genomic regulation by RA, DHA and AA, which could be involved in nuclear signaling events.

Endonuclear lipids in liver cells

Lipids are not only components of cell nucleus membranes, but are also found in the membrane-depleted nuclei where they fulfill special functions. We have investigated the lipid composition of membrane-depleted rat liver nuclei obtained by incubation with low Triton X-100 concentrations of 0.04% and 0.08%, which rendered them unaltered or hardly altered. Under these conditions, 26% of proteins and 22% of phospholipids were recovered. The main phospholipids were phosphatidylcholine > phosphatidylethanolamine > phosphatidylinositol = or > phosphatidylserine and sphingomyelin (in decreasing concentrations). The fatty acid components of total lipids and phosphatidylcholine were mainly unsaturated. Over 40% belonged to the n-6 series (arachidonic > or = 25% and linoleic 15%); approximately 40% corresponded to saturated acids and <10% were monoenoic. Endonuclear phosphatidylcholine was built up by 16 molecular species, the most abundant being 18:0-20:4 (32%), 16:0-20:4 (19%), 16:0-18:2 (13%), and 18:0-18:2 (11%). The fatty acid composition and phosphatidylcholine molecular species distribution in the membrane-depleted nucleus of rat liver showed patterns similar to the whole nucleus, mitochondria, microsomes, and homogenate of the parent liver cells, suggesting that endonuclear lipid pool composition is mainly determined by a liver organ profile.

Retinoic acid-mediated phospholipase A2 signaling in the nucleus

Retinoic acid modulates a wide variety of biological processes including proliferation, differentiation, and apoptosis. It interacts with specific receptors in the nucleus, the retinoic acid receptors (RARs). The molecular mechanism by which retinoic acid mediates cellular differentiation and growth suppression in neural cells remains unknown. However, retinoic acid-induced release of arachidonic acid and its metabolites may play an important role in cell proliferation, differentiation, and apoptosis. In brain tissue, arachidonic acid is mainly released by the action of phospholipase A2 (PLA2) and phospholipase C (PLC)/diacylglycerol lipase pathways. We have used the model of differentiation in LA-N-1 cells induced by retinoic acid. The treatment of LA-N-1 cells with retinoic acid produces an increase in phospholipase A2 activity in the nuclear fraction. The pan retinoic acid receptor antagonist, BMS493, can prevent this increase in phospholipase A2 activity. This suggests that retinoic acid-induced stimulation of phospholipase A2 activity is a retinoic acid receptor-mediated process. LA-N-1 cell nuclei also have phospholipase C and phospholipase D (PLD) activities that are stimulated by retinoic acid. Selective phospholipase C and phospholipase D inhibitors block the stimulation of phospholipase C and phospholipase D activities. Thus, both direct and indirect mechanisms of arachidonic acid release exist in LA-N-1 cell nuclei. Arachidonic acid and its metabolites markedly affect the neurite outgrowth and neurotransmitter release in cells of neuronal and glial origin. We propose that retinoic acid receptors coupled with phospholipases A2, C and D in the nuclear membrane play an important role in the redistribution of arachidonic acid in neuronal and non-nuclear neuronal membranes during differentiation and growth suppression. Abnormal retinoid metabolism may be involved in the downstream transcriptional regulation of phospholipase A2-mediated signal transduction in schizophrenia and Alzheimer disease (AD). The development of new retinoid analogs with diminished toxicity that can cross the blood-brain barrier without harm and can normalize phospholipase A2-mediated signaling will be important in developing pharmacological interventions for these neurological disorders.

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.

Phosphatidyl choline fatty acid remodeling in the hepatic cell nuclei

This study was performed to determine whether fatty acids incorporated into liver cell nuclei phosphatidylcholine (PtdCho) could be remodeled in the isolated nuclear. For this reason, rat liver cell nuclei were incubated in vitro with [1-14C]20:4n-6-CoA. PtdCho molecular species with the highest specific activity had an unsaturated fatty acid at sn-1 and sn-2 positions (20:4-20:4>18:2-20:4>18:1-20:4). 16:0-20:4 and 18:0-20:4 PtdChos showed a minor specific activity. When labeled nuclei were reincubated in the absence of labeled substrate with the addition of cytosol, ATP and CoA, the specific activity of 20:4-20:4, 18:2-20:4 and 18:1-20:4 species decreased, while that of 16:0-20:4 and 18:0-20:4 increased. In conclusion, the asymmetric fatty acid distribution of saturated fatty acids at sn-1 position, and unsaturated fatty acids at sn-2 position of nuclear PtdCho molecular species was re-established by an acyl-CoA-dependent remodeling process.

Nuclear lipids: key signaling effectors in the nervous system and other tissues

Lipids have long been recognized as quantitatively minor components of the nucleus, where they were initially thought to have little functional importance; but they now command growing interest, with recognition of their diverse signaling and modulating properties in that organelle. This applies to the lipid-poor compartments of the nucleoplasm as well as the relatively lipid-rich nuclear envelope. Phosphoglycerides and sphingomyelin, as the predominant lipids, have attracted the most interest among researchers, but some of the less-abundant lipids such as gangliosides, sphingosine, and sphingosine phosphate are now becoming recognized as functionally important nuclear constituents. Among recent advances in this emerging field are detailed findings on the metabolic enzymes that synthesize and catabolize nuclear lipids; the fact that these are localized primarily within the nucleus itself indicates considerable autonomy with respect to lipid metabolism. Current studies suggest several key processes involving RNA and DNA reactivity that are dependent on these lipid-initiated events. Neural cell nuclei have been the subject of such investigations, with results that closely parallel the more numerous studies on nuclei of extraneural cells. This review attempts to outline some of the major findings on nuclear lipids of diverse cell types; results with nonneural nuclei will hopefully provide useful guideposts to further studies of neural systems.

Essential Ca(2+)-independent role of the group IVA cytosolic phospholipase A(2) C2 domain for interfacial activity

The cytosolic Group IVA phospholipase A2 (GIVAPLA2) translocates to intracellular membranes to catalyze the release of lysophospholipids and arachidonic acid. GIVAPLA2 translocation and subsequent activity is regulated by its Ca2+-dependent phospholipid binding C2 domain. Phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2) also binds with high affinity and specificity to GIVAPLA2, facilitating membrane binding and activity. Herein, we demonstrate that GIVAPLA2 possessed full activity in the absence of Ca2+ when PI-4,5-P2 or phosphatidylinositol 3,4,5-trisphosphate were present. A point mutant, D43N, that is unable to bind Ca2+ also had full activity in the presence of PI-4,5-P2. However, when GIVAPLA2 was expressed without its Ca2+-binding C2 domain (DeltaC2), there was no interfacial activity. GIVAPLA2 and DeltaC2 both had activity on monomeric lysophospholipids. DeltaC2, but not the C2 domain alone, binds to phosphoinositides (PIPns) in the same manner as the full-length GIVAPLA2, confirming the location of the PIPn binding site as the GIVAPLA2 catalytic domain. Moreover, proposed PIPn-binding residues in the catalytic domain (Lys488, Lys541, Lys543, and Lys544) were confirmed to be essential for PI-4,5-P2-dependent activity increases. Exploiting the effects of PI-4,5-P2, we have discovered that the C2 domain plays a critical role in the interfacial activity of GIVAPLA2 above and beyond its Ca2+-dependent phospholipid binding.

The presence of ceramidase activity in liver nuclear membrane

This report presents a demonstration of ceramidase activity in the nuclear membrane or envelope of mammalian livers. The products of ceramidase reaction were identified by means of TLC for released fatty acid and HPLC for sphingosine. The ceramidase activity was maximum over a broad neutral to alkaline region ranging from pH 7.0 to 8.8. This activity was inhibited by N-oleoylethanolamine known as a specific inhibitor for ceramidase and by anandamide to a similar extent. The enzymatic study suggests that the nuclear ceramidase has different properties from other ceramidase reported previously. As sphingomyelinase, one of enzymes involved in the sphingomyelin cycle, are known to be present in the nuclear membrane, it is now evident that at least two enzymes involved in the sphingomyelin cycle are present in the nuclear membrane.

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.

Functional role of phospholipids in the nuclear events

This review presents the structural and functional role of phospholipids in chromatin and nuclear matrix as well as the difference in composition and turnover compared to those present in the nuclear membrane. Nuclei have a very active lipid metabolism which seems to play an important role in the transduction of the signals to the genome in response to agonists acting at the plasma membrane level. The evidence on the presence of phospholipid-calcium-dependent protein kinase C (PKC) in nuclei and enzymes of phospholipids turnover is given. Protein kinase C interacts with nuclear phosphoinositol and sphingomyelin cycles products. This fact evidences about possibility that signal transduction events could also occur at the nuclear level during induction of cell proliferation, differentiation and apoptosis.

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.

Nuclear localization of neutral sphingomyelinase 1: biochemical and immunocytochemical analyses

To examine the intracellular localization of neutral sphingomyelinase 1 (nSMase 1), a rabbit polyclonal antibody was raised against a recombinant form of the enzyme expressed in E. coli. It has been reported that, in rat liver or in ascites hepatoma AH7974, high activity of neutral sphingomyelinase (SMase) is found at the plasma membrane, with a lesser but significant amount in nucleus and cytoplasm. The biochemical properties, dithiothreitol requirement and high salt concentration dependency, of cloned and expressed nSMase 1 resemble those of previously described nuclear neutral SMase of AH7974. The present study was therefore focused on the nuclear localization of this enzyme. Western blotting of subcellular fractions using anti-rat nSMase 1 antibody revealed most nSMase 1 to be associated with the nuclei and some with microsomes, but not with plasma membranes. Consistently, neutral SMase activity in nuclear extract was immunoprecipitated by the antibody, while that of plasma membranes was not. The results indicate that nSMase 1 mainly resides in the nucleus and may thus differ from neutral SMase in plasma membrane. On gel-filtration column chromatography of nuclear extract, the profile of neutral SMase activity corresponded well with immunoreactive protein bands on western blotting, suggesting that a large part of nuclear neutral SMase may be nSMase 1. Removal of the nuclear envelope by treatment with Triton X-100 did not significantly decrease the amount of nuclear nSMase 1, and western blotting of subnuclear fractions (i.e. nuclear envelope, chromatin, and nuclear matrix) revealed nSMase 1 signal exclusively in the nuclear matrix. Immunocytochemistry with AH7974, as well as rat fibroblast cell line 3Y1, demonstrated nSMase 1 to be localized mainly in the nucleus, with some in the cytoplasm. Moreover, immuno-electron microscopy clearly showed the signal of nSMase 1 to be more dense in the nucleus than in the cytoplasm of AH7974.

The expanding superfamily of phospholipase A(2) enzymes: classification and characterization

The phospholipase A(2) (PLA(2)) superfamily consists of a broad range of enzymes defined by their ability to catalyze the hydrolysis of the middle (sn-2) ester bond of substrate phospholipids. The hydrolysis products of this reaction, free fatty acid and lysophospholipid, have many important downstream roles, and are derived from the activity of a diverse and growing superfamily of PLA(2) enzymes. This review updates the classification of the various PLA(2)'s now described in the literature. Four criteria have been employed to classify these proteins into one of the 11 Groups (I-XI) of PLA(2)'s. First, the enzyme must catalyze the hydrolysis of the sn-2 ester bond of a natural phospholipid substrate, such as long fatty acid chain phospholipids, platelet activating factor, or short fatty acid chain oxidized phospholipids. Second, the complete amino acid sequence of the mature protein must be known. Third, each PLA(2) Group should include all of those enzymes that have readily identifiable sequence homology. If more than one homologous PLA(2) gene exists within a species, then each paralog should be assigned a Subgroup letter, as in the case of Groups IVA, IVB, and IVC PLA(2). Homologs from different species should be classified within the same Subgroup wherever such assignments are possible as is the case with zebra fish and human Group IVA PLA(2) orthologs. The current classification scheme does allow for historical exceptions of the highly homologous Groups I, II, V, and X PLA(2)'s. Fourth, catalytically active splice variants of the same gene are classified as the same Group and Subgroup, but distinguished using Arabic numbers, such as for Group VIA-1 PLA(2) and VIA-2 PLA(2)'s. These four criteria have led to the expansion or realignment of Groups VI, VII and VIII, as well as the addition of Group XI PLA(2) from plants.

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.

A possible role of nuclear ceramide and sphingosine in hepatocyte apoptosis in rat liver

BACKGROUND/AIMS

Portal vein branch ligation induces apoptosis of hepatocytes in the ligated lobes in rat liver. Sphingomyelin degradation was studied during the process to evaluate its possible involvement in apoptosis in vivo.

METHODS

DNA scissions were detected by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and an agarose gel electrophoresis of DNA. Using both ligated and non-ligated lobes, we measured activities of sphingomyelin degradation enzymes and contents of their products in purified nuclei and plasma membrane.

RESULTS

DNA fragmentation was detectable in the ligated lobes at 90 min after the portal vein branch ligation by gel electrophoresis. At 15 h after the ligation, 27% of hepatocytes became TUNEL-positive. Prior to the onset of apoptosis, the activity of neutral sphingomyelinase increased in the nuclei of hepatocytes in ligated lobes (30 min after the ligation). The increase in sphingomyelinase paralleled its reaction product, ceramide. This was followed by the elevation of ceramidase activity in nuclei (60 min after the ligation) in association with an increase of its reaction product, sphingosine. Activities of these two enzymes and their products increased for at least 90 min. These changes were not observed in nuclei of the non-ligated lobes, or in the plasma membranes from either ligated or non-ligated lobes.

CONCLUSIONS

These results, specific to the liver where apoptosis is being generated, suggest that nuclear sphingomyelin breakdown with an accumulation of ceramide and/or sphingosine in nuclei may induce the apoptosis of hepatocytes in vivo.

Phosphatidylinositol 3-kinase in HL-60 nuclei is bound to the nuclear matrix and increases during granulocytic differentiation

We have used HL-60 leukemia cells to investigate phosphatidylinositol 3-kinase (PI 3-K) during granulocytic differentiation at the nuclear level. Nuclei of HL-60 cells showed a constitutive presence of PI 3-K that increased when cells were treated with differentiating doses of ATRA. PI 3-K was also detected tightly bound to nuclear matrices of HL-60 cells, isolated by nuclease treatment and high salt extraction. Four days of ATRA treatment induced a striking increase of nuclear matrix bound PI 3-K. In situ morphological analysis by confocal microscopy showed the translocation of PI 3-K to the nucleus and to the subnuclear fractions. PI 3-K enzymatic activity was stimulated during the granulocytic differentiation process and parallelled the increase in content of nuclei and subnuclear fractions. PI 3-K activity was recovered in nuclei also without the addition of exogenous substrates, consistent with the presence of both substrates and enzyme in the nucleus. These results indicate that specific intracellular localization of PI 3-K determines the production of different phosphoinositides in the sites of the enzyme translocation, and suggest that 3-phosphoinositide metabolism may play a specific role in the nucleus, candidating PI 3-K as a key enzyme in promoting granulocytic differentiation of HL-60 cells.

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.

Nuclear location of PLA2-I in proliferative cells

We have previously demonstrated that pancreatic PLA2 (PLA2-I) stimulates the proliferation of UIII cells, a stromal cell line derived from normal rat uterus. In order to gain further insight into the mechanism of action of PLA2-I, we have investigated the intracellular processing of PLA2-I. Either highly proliferative or growth arrested UIII cells were analyzed. Growth arrested cells were obtained from a contact inhibited monolayer or from aristolochic acid-treated cultures. Using cellular fractionation, western blotting, immunocytochemistry and confocal microscopy, we demonstrate that endogenous PLA2-I was mainly located in the nucleus in highly proliferative cells whereas its location was cytoplasmic in non proliferative cells. When non confluent UIII cells were incubated with nanomolar amounts of exogenous PLA2-I, the enzyme was internalized and, in the majority of cells, appeared within the nucleus. Both internalization and nuclear location of exogenous PLA2-I were suppressed by the addition of aristolochic acid to the culture medium. Binding experiments performed on purified nuclear preparations showed the presence of specific cooperative binding sites for PLA2-I. Collectively our data suggest that the proliferative effect exerted by pancreatic PLA2 in UIII cells is mediated by a direct interaction of the enzyme at the nuclear level. Putative mechanisms and targets are discussed.

Metabolism and possible compartmentalization of inositol lipids in isolated rat-liver nuclei

(1) The removal of the nuclear envelope from isolated rat-liver nuclei by washing with Triton X-100 (TX-100) was assessed by electron microscopy. All the envelope was removed by 0.04% (w/v) TX-100. (2) After this removal, phosphorylation of inositol lipids and diacylglycerol (DAG) from [gamma-32P]ATP still occurs, despite the near complete absence of detectable (by mass assay) DAG and PtdIns. This suggests that the majority of these two lipids in nuclei are present in the nuclear membrane, but the small amounts remaining after extraction, defined as intranuclear, are available for phosphorylation by lipid kinases (36% for DAG and 24% for PtdIns respectively, when expressed as a percentage of incorporation of intact nuclei). (3) PtdIns(4,5)P2 did not follow the same pattern as PtdIns and DAG; after removal of the nuclear membrane, 40% of the mass of this lipid was left in the nucleus. Moreover, a similar amount of PtdIns(4,5)P2 was also resistant to extraction with even higher concentrations of detergent, suggesting that PtdIns(4,5)P2 has a discrete intranuclear location, probably bound to nuclear proteins. (4) Addition of exogenous substrates, PtdIns, PtdIns(4)P and DAG, to membrane-depleted nuclei resulted in reconstitution of the majority of lipid phosphorylations from [gamma-32P]ATP (70%, 90% and 94% of intact nuclei respectively), suggesting a predominantly intranuclear location for the respective kinases. (5) Nuclei also showed phosphomonoesterase and phosphatidic acid hydrolase activity; dephosphorylation of pre-radiolabelled PtdIns(4)P, PtdIns(4,5)P2 and phosphatidic acid was observed when [gamma-32P]ATP was removed. However, some of the radioactivity was apparently resistant to these enzymes, suggesting the existence of multiple pools of these lipids. (6) Addition of excess non-radiolabelled ATP to nuclei pre-labelled with [gamma-32P]ATP resulted in an initial increase in the label in PtdIns(4,5)P2, implying a precursor-product relationship between the radiolabelled pools of PtdIns(4)P and PtdIns(4,5)P2. This was confirmed by analysis of the incorporation of 32P into the 4'-phosphate group of PtdIns(4)P and the individual 4'- and 5'-phosphate groups of PtdIns(4,5)P2. The data from these experiments also indicated that PtdIns(4,5)P2 can be produced from a pre-existing pool of PtdIns(4)P, as well as de novo from PtdIns. (7) Taken together our data suggest that isolated rat-liver nuclei have an intranuclear inositol lipid metabolism mechanism utilizing enzymes and substrates equivalent to those found in cytosol and plasma membrane, and that there may be some, but not complete, compartmentalization of the components of the nuclear inositol cycle.

Nuclear ADP-ribosylation factor (ARF)- and oleate-dependent phospholipase D (PLD) in rat liver cells. Increases of ARF-dependent PLD activity in regenerating liver cells

Two forms of phospholipase D (PLD) have been found to be present in nuclei isolated from rat hepatocytes by measuring phosphatidylbutanol produced from exogenous radiolabeled phosphatidylcholine in the presence of butanol. In nuclear lysates from either rat liver or ascites hepatoma AH 7974 cells, the PLD activity was markedly stimulated by a recombinant ADP-ribosylation factor (rARF) in the presence of the guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and phosphatidylinositol 4, 5-bisphosphate. ATP and phorbol-12-myristate 13-acetate had no synergistic effect on this PLD activity. On the other hand, the nuclear PLD was stimulated by unsaturated fatty acids, especially by oleic acid. The ARF-dependent nuclear PLD activity was increased in the S-phase of the regenerating rat liver after partial hepatectomy and also was much higher in AH 7974 cells than in the resting rat liver. In contrast, the levels of the oleate-dependent PLD activity remained constant throughout the cell cycle in liver regeneration. The intranuclear levels of the stimulating proteins of the nuclear PLD activity, e.g. ARF, RhoA, and protein kinase Cdelta increased in the S-phase of the regenerating liver. These results suggested that the nuclear ARF-dependent PLD activity may be associated with cell proliferation.

Purification and characterization of nuclear alkaline phospholipase A2 in rat ascites hepatoma cells

The alkaline phospholipase A2 (PLA2) was purified from nuclei of rat ascites hepatoma cells (AH7974) by column chromatography with a Sephacryl S-300 column and an immunoadsorbent using anti-group II PLA2 monoclonal antibody. From these two columns, the alkaline PLA2 was eluted in parallel with a 17-kDa protein which is reactive to another anti-group II PLA2 polyclonal antibody. Approximately 80% of nuclear PLA2 was inhibited by this antibody. The alkaline PLA2 was found in association with the chromatin fraction among subnuclear fractions. By an immunocytochemical staining, the nuclei of AH7974 were stained more strongly than other parts of cells with anti-group II PLA2 antiserum.

Phorbol 12-myristate 13-acetate-mediated signalling in murine bone marrow cells

Phorbol 12-myristate 13-acetate (PMA)-mediated signalling was investigated in relation to the ability of murine (CBA) bone marrow cells to form colonies in vitro. Treatment of marrow cells with PMA did not influence the 1,2-diacylglycerol or cyclic AMP concentrations, the intracellular Ca2+ concentration or phospholipase D activity. PMA increased particulate phospholipase A2 (PLA2) activity, lysophosphatidylcholine formation and arachidonic acid release from bone marrow cells; these effects were abolished when cells were pretreated with the putative PLA2 inhibitors heparin and mepacrine. While indomethacin and nordihydroguaiaretic acid inhibited either the cyclo-oxygenase or lipoxygenase pathway of arachidonic acid metabolism, as measured by their products prostaglandin E2 and leukotriene B4, they did not influence PMA-mediated PLA2 activation or translocation of protein kinase C (PKC) from the soluble to the particulate fraction. Treatment of cells with PMA increased the amounts of membrane-bound alpha, beta, delta, epsilon and zeta isoforms of PKC in bone marrow cells. Pretreatment of cells with PLA2 inhibitors reduced the amount of membrane-bound PKC-zeta in unstimulated cells and diminished PMA-induced translocation of PKC-zeta to membranes without affecting other PKC isoforms. This effect could be overcome by exogenous addition of arachidonic acid, suggesting that PKC-zeta may operate downstream of the activated PLA2. On the other hand, wortmannin, an inhibitor of phosphatidylinositol 3-kinase, did not influence the amount of PKC-zeta associated with particulate fractions in control cells and could not abolish the PMA-mediated translocation of this isoform. Short-term exposure (45 min) of bone marrow cells to PMA, phorbol 12,13-dibutyrate or arachidonic acid increased the number of colonies formed over 7 days in a methylcellulose-based culture in vitro. The effects of PMA, but not those of arachidonic acid, could be prevented by putative PLA2 inhibitors. This suggests that PMA-mediated activation of conventional PKCs and novel PKCs leads to PLA2 activation which, by releasing arachidonic acid from phospholipids, activates PKC-zeta. This signalling pathway appears to be mitogenic for bone marrow cells.

Lipid-dependent nuclear signalling: morphological and functional features

Enzymes involved in lipid metabolism exist within the nucleus and are responsive to external stimuli. In particular, the kinases which phosphorylate phosphatidylinositol and phosphatidylinositol-4-monophosphate have been demonstrated in nuclei of both undifferentiated and differentiated Friend cells and of quiescent Swiss 3T3 cells as well as of those exposed to insulin-like growth factor I. Besides the lipid kinases, also the phosphoinositidases C (PIC) are active inside the nucleus. In Swiss 3T3 cells the nuclear PIC beta 1 is activated and its activation by IGF-I temporally precedes the translocation to the nucleus of protein kinase C. In Friend cell nuclei, on the other hand, when erythroid differentiation is induced, the PIC beta 1 activity is reduced. Another aspect of the nuclear signalling transduction system which appears quite interesting is its actual localization at subcellular level. By using electron microscope immunogold labelling, the nuclear PIC isoforms (the beta 1 isoform in Swiss 3T3 cells, the beta 1 and gamma 1 in Friend cells) are localized mainly in the interchromatin domains. This localization has been further confirmed on in situ matrix preparations of 3T3 cells in which PIC beta 1 is associated with the inner nuclear matrix but not with the nuclear pore-lamina complex. Colocalization experiments indicate that nuclear PIC beta 1 is present in sites in which both nuclear phospholipids and PKC can be detected, while the cytoplasmic PIC gamma 1 can be identified in close association with cytoskeletal filaments identified by anti-actin antibodies. The precise localization of the different PIC isoforms strongly indicates that the signal transduction system operating at the nuclear level may be part of a cross-talk between the cytoplasm and the nucleus controlling either cell proliferation or differentiation.

Sphingosine-mediated membrane association of DNA and its reversal by phosphatidic acid

Resonance energy transfer was measured between egg phosphatidylcholine liposomes containing the intramolecular excimer forming pyrene-labelled phospholipid analogue 1,2-bis[pyren-1-(-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC) as a donor and DNA-bound adriamycin as an acceptor. Membrane association of DNA turned out to be critically dependent on the presence of sphingosine in the liposomes. Identical result was obtained by measuring the extent of quenching of the fluorescent DNA-bound dye Hoechst 33258 due to energy transfer to the lipophilic stain Nile Red incorporated in egg phosphatidylcholine liposomes containing varying amounts of sphingosine. The attachment of DNA to sphingosine-containing membranes could be reversed by the further inclusion of the negatively charged phosphatidic acid up to approximately 1:2 PA/sphingosine molar ratio in the liposomes, thus suggesting the involvement of electrostatic interactions. Differential scanning calorimetry measurements confirmed a lack of association between DNA and dimyristoylphosphatidylcholine liposomes. Instead drastic changes were produced by DNA in the heat capacity scans measured for liposomes also incorporating sphingosine. Fluorescence microscopy revealed an extensive aggregation of sphingosine containing pyrene-phosphatidylcholine-labelled egg phosphatidylcholine liposomes in the presence of DNA. Together with other available data on the effects of sphingosine, the present findings suggest that sphingosine could directly alter the chromatin structure. Accordingly, such alterations may contribute to the control of replication and gene expression.

PLA2 activity in rat liver nuclei

1. Subcellular fractions of rat liver were assayed for PLA2 activity. 2. The PLA2 assay measures the release of [3H]oleic acid from phospholipids, using labeled E. coli as substrate. 3. Nuclear fractions contained PLA2 activity, which was Ca2+ dependent and could not be explained from mitochondria, microsomal or plasma membrane contamination. 4. The Vmax value of nuclear PLA2 is 0.30 +/- 0.04 pmol oleic acid/min/mg protein; its Km value is 0.86 +/- 0.12 microM, similar to that of mitochondrial PLA2. 5. We conclude that rat liver nuclei contain PLA2 activity.

RFP is a DNA binding protein associated with the nuclear matrix

We reported that the RFP gene encodes a protein with putative zinc finger domains and was involved in the activation of the ret proto-oncogene. To further characterize the RFP protein, we developed a polyclonal antibody against the product synthesized from a fragment of the RFP cDNA expressed in Escherichia coli. Western blot analysis showed that RFP was identified as a 58 kDa protein in cell lysates from four human and rodent cell lines and from mouse testis. In addition, a unique 68 kDa protein was detected in the testis. Using AH7974 (rat ascites hepatoma) and Raji (human Burkitt lymphoma) cells, we demonstrated strong association of RFP with the nuclear matrix. Furthermore, RFP solubilized from the nuclear matrix had DNA-binding activity although it appears to bind more preferentially to double-stranded DNA than to single-stranded DNA. These results thus suggest that RFP may play a role in molecular processes which occur in the nuclear matrix.

Phospholipids in the nucleus--metabolism and possible functions

Most of the phospholipids in the nuclear envelope are contained in the double nuclear membrane, and this has an active lipid metabolism consistent with its origins as a component of the endoplasmic reticular system. However, even after removal of the nuclear membrane with detergents, some phospholipids, mostly of unknown location and function, remain. Amongst these are all of the components of what appears to be a nuclear polyphosphoinositide signalling system, distinct from the well-established inositide pathway found in the plasma membrane. The consequences for nuclear function of the activation of these two inositide pathways are discussed, with a detailed consideration of proposed intranuclear functions for protein kinase C, and the maintenance of nuclear Ca2+ homoeostasis.

Morphological and biochemical evidence for partial nuclear localization of annexin 1 in endothelial cells

Using immunofluorescence, an affinity-purified anti-annexin-1 polyclonal antibody showed both cytoplasmic and nuclear staining, whereas antibodies against annexins 2, 5 and 6 labelled almost exclusively the cytoplasm of cultured endothelial cells. This was further confirmed by immunogold labelling and electron microscopy using a monoclonal antibody, annexin 1 being detected close to the plasma membrane, in the cytoplasm, as well as inside the nucleus. Finally, using immunoblotting, purified nuclei were shown to contain annexin 1, which was not removed by EDTA treatment. These data open some new perspectives in the understanding of annexin function, including possible involvement in nucleoskeleton dynamics and regulation of proliferation through cell signalling.

Properties of plasma membrane-induced amylase release from rat parotid secretory granules: effects of Ca2+ and Mg-ATP

A secretory granular fraction (SG) and a plasma membrane rich fraction (PM) have been isolated from rat parotid gland by differential and Percoll gradient centrifugation. With these two fractions, a cell-free interaction system has been reconstituted to clarify the exocytotic interaction between the secretory granules and plasma membranes, and the conditions of amylase release from SG have been characterized in vitro. The addition of PM into this assay system induced a rapid and transient release of amylase from SG. Some other membranes such as erythrocyte ghosts also mimicked the effect of PM. This release was increased by Ca2+, but was not completely blocked by EGTA. Simultaneous addition of 1 mM ATP with 1 mM MgCl2 (Mg-ATP) in the presence of Ca2+ reduced this release. However, in spite of the existence of Mg-ATP, the stimulation of PM-induced amylase release was caused by Ca2+ in a concentration-dependent manner (10(-7)-10(-3) M). These results suggest that Ca2+ and Mg-ATP should participate as important regulators in the exocytotic interaction between secretory granules and plasma membranes in this system. Furthermore, the differences between our system and intact cells are also discussed.

Ca(2+)-induced reversible translocation of phospholipase A2 between the cytosol and the membrane fraction of rat liver macrophages

In cell-free extracts of rat liver macrophages (Kupffer cells) phospholipase A2 was found to be rapidly associated with the particulate fraction in a Ca(2+)-dependent manner at Ca2+ concentrations of 0.1-1.0 microM. This is also the range of the levels of intracellular Ca2+ reported for basal and various stimulated conditions. After translocation, phospholipase A2 could be released from the membranes in the presence of Ca2+ chelators, increasing the specific activity of phospholipase A2 in the supernatant fraction. These findings support the view that translocation is a regulatory mechanism of phospholipase A2 by bringing the enzyme to its substrate. Unlike the situation with protein kinase C, Mg2+ exerted little effect on phospholipase A2 translocation, indicating that this process is regulated in vivo mainly by fluctuations of the intracellular Ca2+ content.

Growth-associated changes in fatty acid compositions of nuclear phospholipids of liver cells

To know the possible relationships between nuclear phospholipids and cell proliferation, we have extensively analyzed phospholipids extracted from the nuclei of rat hepatic cells at various growth states. The content of phospholipid in nuclei as well as its composition was similar among liver cells tested, i.e., the regenerating rat livers (28 h, post-hepatectomy), sham-operated or non-treated control livers, and rat ascites hepatoma, AH7974 cells. In contrast, the fatty acid compositions of phospholipids differed from each other among these cells. At the 2-position of phospholipids in the regenerating liver nuclei at 28 h after partial hepatectomy, 18:1 (oleic acid) increased transiently at the expense of 20:4 (arachidonic acid) and 22:6 (docosahexaenoic acid), compared with those in the sham-operated control nuclei. This change in fatty acid composition was commonly observed throughout all phospholipids analyzed, i.e., phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS). On the other hand, the change at 1-position was rather limited: in the regenerating liver nuclei (28 h), 18:1 increased only in PC at the expense of 18:0 (stearic acid). The similar and more marked deviation at the 2-position was observed with AH7974 nuclei it contained approximately 2-times more of 18:1 in PC, PE and PI than regenerating liver nuclei (28 h), and the decreased levels of 20:4 and/or 22:6. It should be noted that there were significant differences in the fatty acid compositions of PE and PS between sham-operated and non-treated controls. So, the sham-operated rat is the appropriate control for proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma membrane-specific phospholipase A1 activation by nitrogen dioxide in pulmonary artery endothelial cells

Nitrogen dioxide (NO2), an environmental oxidant, alters the plasma membrane structure and function of pulmonary artery endothelial cells through peroxidative injury. Because perioxidative injury can activate membrane phospholipases and alter phospholipid composition of membranes, we evaluated the effects of NO2 exposure on phospholipase A1 (PLA1), phospholipase A2 (PLA2), and diacylglycerol lipase (DG lipase) activities in pulmonary artery endothelial cell plasma, mitochondrial, and microsomal membranes. We also evaluated the effect of NO2 exposure on the phospholipid composition of plasma membranes of these cells. Exposure to 5 ppm NO2 for 48 hr resulted in a significant (p less than 0.01) increase in PLA1 activity in plasma membranes but not in mitochondrial or microsomal membranes of pulmonary artery endothelial cells, whereas PLA2 and DG lipase activities were comparable to controls in all membranes. As a result of PLA1 activation, the total phospholipid content of the plasma membranes of NO2-exposed cells was significantly (p less than 0.01) reduced compared to controls. Phosphatidylethanolamine (PE) content was reduced (p less than 0.05), whereas lyso-PE (LPE), a product of PLA1 hydrolysis of PE, as well as phosphatidylserine (PS) contents were increased (p less than 0.01 for both LPE and PS) in the plasma membranes of NO2-exposed cells. Incorporation of exogenous PS into pulmonary artery endothelial cells mimicked the stimulatory effect of NO2 on PLA1 activity. These results demonstrate that NO2 specifically reacts with the plasma membrane component of pulmonary artery endothelial cells, causing specific activation of PLA1. The NO2-induced increase of PS in the plasma membranes appears to be responsible for the specific activation of PLA1 in pulmonary artery endothelial cells.

Inhibition of nuclear T3 binding via PLA2-induced release of fatty acids from nuclear membranes

This study was undertaken to explore putative regulatory mechanisms involved in the inhibition of nuclear T3 binding (INB) by fatty acids. Ether extracts of intact rat liver nuclei contained INB-activity. Removement of the nuclear membrane resulted in the loss of INB-activity of the nuclei. Incubation of intact nuclei with phospholipase A2 increased nuclear INB-activity in a time- and dose-dependent manner; this was correlated with a rise of free fatty acid concentration in the ether extract. We conclude that fatty acids present in the nuclear membrane can be released by phospholipase A2, and are capable of inhibiting nuclear T3 binding.

Nerve growth factor stimulation of arachidonic acid release from PC12 cells: independence from phosphoinositide turnover

The effect of nerve growth factor on the metabolism of arachidonic acid and the hydrolysis of phosphatidylinositol in PC12 cells was examined. Addition of nerve growth factor to PC12 cells isotopically labeled with [3H]arachidonic acid caused an increased release of radioactivity. In a similar manner, treatment of PC12 cells prelabeled with [3H]inositol increased inositol monophosphate accumulation in the presence of LiCl. Stimulation of [3H]arachidonic acid release by nerve growth factor was concentration dependent, attaining a maximum at 0.5 nM. Concentrations of nerve growth factor above 0.5 nM caused less than maximal stimulation. In contrast, nerve growth factor-stimulated accumulation of [3H]inositol monophosphate exhibited a sigmoidal dose-response curve with an apparent maximum at 8 nM. Increased accumulation of [3H]inositol monophosphate could be detected as early as 60 s after nerve growth factor addition, whereas nerve growth factor-stimulated release of [3H]arachidonic acid was not observed until 5 min after nerve growth factor treatment. The nerve growth factor-stimulated release of [3H]arachidonic acid was independent of extracellular calcium concentration. Increased [3H]inositol monophosphate accumulation elicited by nerve growth factor was dependent on the presence of extracellular calcium. These results suggest that the increased metabolism of arachidonic acid and the enhanced hydrolysis of phosphatidylinositol are separately regulated by nerve growth factor.

Anionic phospholipids stimulate an arachidonoyl-hydrolyzing phospholipase A2 from macrophages and reduce the calcium requirement for activity

Mechanisms involved in regulating the activity of intracellular phospholipase A2 enzymes that function in eicosanoid and platelet-activating factor production are poorly understood. The properties of the substrate in the membrane may play a role in modulating phospholipase A2 activity. In this study, the effect of anionic phospholipids, diacylglycerol (DAG) and phosphatidylethanolamine (PE) on the activity of a partially purified, intracellular, arachidonoyl-hydrolyzing phospholipase A2 from the macrophage cell line, RAW 264.7 was studied. For these experiments phospholipase A2 activity was assayed in the presence of 1 microM calcium by measuring the hydrolysis of [3H]arachidonic acid from sonicated dispersions of the ether-linked substrate, 1-O-hexadecyl-2[3H]arachidonoylglycerophosphocholine. All the anionic phospholipids tested, including phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2), stimulated phospholipase A2 activity. At the lowest concentration of anionic phospholipids tested. PIP2 was the most stimulatory, resulting in a 7-fold increase in phospholipase A2 activity at 1 mol%. Co-dispersion of either DAG or PE with the substrate also induced a dose-dependent increase in phospholipase A2 activity, whereas sphingomyelin was inhibitory suggesting that the phospholipase A2 more readily hydrolyzed the ether-linked substrate when there was a decrease in the packing density of the bilayer. PIP2, together with either DAG or PE, synergistically stimulated phospholipase A2 activity by about 20-fold, and dramatically decreased the calcium concentration (from mM to nM) required for full activity of the enzyme. The results of this study demonstrate that the presence of anionic phospholipids and the packing characteristics of the bilayer can have pronounced effects on the activity and calcium requirement of an intracellular, arachidonoyl-hydrolyzing phospholipase A2 from macrophages.

Rapid actions of 1 alpha,25-dihydroxyvitamin D3 on Ca2+ and phospholipids in isolated rat liver nuclei

The effects of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) on Ca2+ levels and phospholipid metabolism were studied in isolated nuclei prepared from rat liver. Nuclear Ca2+ concentration was estimated with the fluorescent indicator Fura 2. In agreement with previous reports, ATP (1 mM) produced a rapid increase in nuclear Ca2+ from 188 +/- 25 to 593 +/- 121 nM. Exposure to 1 alpha,25-(OH)2D3 (20 nM) also produced a rapid increase in nuclear Ca2+ to 402 +/- 71 nM. The 1 beta epimer of 1 alpha,25-(OH)2D3 had no effect. Nuclear phosphatidylinositol was labeled by incubation with [gamma-32P]ATP for 3 h. 1 alpha,25-(OH)2D3 produced a two-fold increase in [32P]lysophosphatidylinositol (LPI) within 5 min from 44 +/- 11 to 87 +/- 19 cpm/2.5 x 10(7) nuclei. 1 beta,25-(OH)2D3 had no effect on [32P]LPI production. Exposure of nuclei to exogenous LPI (15 microM) produced an instantaneous increase in nuclear Ca2+ to 372 +/- 81 nM, comparable to ATP and 1 alpha,25-(OH)2D3. The rapid effects of 1 alpha,25-(OH)2D3 on phospholipid metabolism and Ca2+ in isolated nuclei suggest that the steroid may exert effects distinct from the well-characterized receptor-mediated changes in gene expression.