Nuclei contain two differentially regulated pools of diacylglycerol

Nuclear Localization of Diacylglycerol Kinase Alpha in K562 Cells Is Involved in Cell Cycle Progression

Phosphatidylinositol (PI) signaling is an essential regulator of cell motility and proliferation. A portion of PI metabolism and signaling takes place in the nuclear compartment of eukaryotic cells, where an array of kinases and phosphatases localize and modulate PI. Among these, Diacylglycerol Kinases (DGKs) are a class of phosphotransferases that phosphorylate diacylglycerol and induce the synthesis of phosphatidic acid. Nuclear DGKalpha modulates cell cycle progression, and its activity or expression can lead to changes in the phosphorylated status of the Retinoblastoma protein, thus, impairing G1/S transition and, subsequently, inducing cell cycle arrest, which is often uncoupled with apoptosis or autophagy induction. Here we report for the first time not only that the DGKalpha isoform is highly expressed in the nuclei of human erythroleukemia cell line K562, but also that its nuclear activity drives K562 cells through the G1/S transition during cell cycle progression. J. Cell. Physiol. 232: 2550-2557, 2017. © 2016 Wiley Periodicals, Inc.

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.

Quantitative profiling of the endonuclear glycerophospholipidome of murine embryonic fibroblasts

A reliable method for purifying envelope-stripped nuclei from immortalized murine embryonic fibroblasts (iMEFs) was established. Quantitative profiling of the glycerophospholipids (GPLs) in envelope-free iMEF nuclei yields several conclusions. First, we find the endonuclear glycerophospholipidome differs from that of bulk membranes, and phosphatidylcholine (PtdCho) and phosphatidylethanolamine species are the most abundant endonuclear GPLs by mass. By contrast, phosphatidylinositol (PtdIns) represents a minor species. We also find only a slight enrichment of saturated versus unsaturated GPL species in iMEF endonuclear fractions. Moreover, much lower values for GPL mass were measured in the iMEF nuclear matrix than those reported for envelope-stripped IMF-32 nuclei. The collective results indicate that the nuclear matrix in these cells is a GPL-poor environment where GPL occupies only approximately 0.1% of the total nuclear matrix volume. This value suggests GPL accommodation in this compartment can be satisfied by binding to resident proteins. Finally, we find no significant role for the PtdIns/PtdCho-transfer protein, PITPα, in shuttling PtdIns into the iMEF nuclear matrix.

Inositol Hexakisphosphate Kinase 1 (IP6K1) Regulates Inositol Synthesis in Mammalian Cells

myo-Inositol, the precursor of all inositol compounds, has pivotal roles in cell metabolism and signaling pathways. Although physiological studies indicate a strong correlation between abnormal intracellular inositol levels and neurological disorders, very little is known about the regulation of inositol synthesis in mammalian cells. In this study, we report that IP6K1, an inositol hexakisphosphate kinase that catalyzes the synthesis of inositol pyrophosphate, regulates inositol synthesis in mammalian cells. Ip6k1 ablation led to profound changes in DNA methylation and expression of Isyna1 (designated mIno1), which encodes the rate-limiting enzyme inositol-3-phosphate synthase. Interestingly, IP6K1 preferentially bound to the phospholipid phosphatidic acid, and this binding was required for IP6K1 nuclear localization and the regulation of mIno1 transcription. This is the first demonstration of IP6K1 as a novel negative regulator of inositol synthesis in mammalian cells.

Nuclear diacylglycerol lipase-α in rat brain cortical neurons: evidence of 2-arachidonoylglycerol production in concert with phospholipase C-β activity

In this report, we describe the localization of diacylglycerol lipase-α (DAGLα) in nuclei from adult cortical neurons, as assessed by double-immunofluorescence staining of rat brain cortical sections and purified intact nuclei and by western blot analysis of subnuclear fractions. Double-labeling assays using the anti-DAGLα antibody and NeuN combined with Hoechst staining showed that only nuclei of neuronal origin were DAGLα positive. At high resolution, DAGLα-signal displayed a punctate pattern in nuclear subdomains poor in Hoechst's chromatin and lamin B1 staining. In contrast, SC-35- and NeuN-signals (markers of the nuclear speckles) showed a high overlap with DAGLα within specific subdomains of the nuclear matrix. Among the members of the phospholipase C-β (PLCβ) family, PLCβ1, PLCβ2, and PLCβ4 exhibited the same distribution with respect to chromatin, lamin B1, SC-35, and NeuN as that described for DAGLα. Furthermore, by quantifying the basal levels of 2-arachidonoylglycerol (2-AG) by liquid chromatography and mass spectrometry (LC-MS), and by characterizing the pharmacology of its accumulation, we describe the presence of a mechanism for 2-AG production, and its PLCβ/DAGLα-dependent biosynthesis in isolated nuclei. These results extend our knowledge about subcellular distribution of neuronal DAGLα, providing biochemical grounds to hypothesize a role for 2-AG locally produced within the neuronal nucleus.

A novel light-dependent activation of DAGK and PKC in bovine photoreceptor nuclei

In this work, we describe a selective light-dependent distribution of the lipid kinase 1,2-diacylglycerol kinase (EC 2.7.1.107, DAGK) and the phosphorylated protein kinase C alpha (pPKCα) in a nuclear fraction of photoreceptor cells from bovine retinas. A nuclear fraction enriched in small nuclei from photoreceptor cells (PNF), was obtained when a modified nuclear isolation protocol developed by our laboratory was used. We measured and compared DAGK activity as phosphatidic acid (PA) formation in PNF obtained from retinas exposed to light and in retinas kept in darkness using [γ-(32)P]ATP or [(3)H]DAG. In the absence of exogenous substrates and detergents, no changes in DAGK activity were observed. However, when DAGK activity assays were performed in the presence of exogenous substrates, such as stearoyl arachidonoyl glycerol (SAG) or dioleoyl glycerol (DOG), and different detergents (used to make different DAGK isoforms evident), we observed significant light effects on DAGK activity, suggesting the presence of several DAGK isoforms in PNF. Under conditions favoring DAGKζ activity (DOG, Triton X-100, dioleoyl phosphatidylserine and R59022) we observed an increase in PA formation in PNF from retinas exposed to light with respect to those exposed to darkness. In contrast, under conditions favoring DAGKɛ (SAG, octylglucoside and R59022) we observed a decrease in its activity. These results suggest different physiological roles of the above-mentioned DAGK isoforms. Western blot analysis showed that whereas light stimulation of bovine retinas increases DAGKζ nuclear content, it decreases DAGKɛ and DAGKβ content in PNF. The role of PIP2-phospholipase C in light-stimulated DAGK activity was demonstrated using U73122. Light was also observed to induce enhanced pPKCα content in PNF. The selective distribution of DAGKζ and ɛ in PNF could be a light-dependent mechanism that in vertebrate retina promotes selective DAG removal and PKC regulation.

Identification of nuclear phosphatidylinositol 4,5-bisphosphate-interacting proteins by neomycin extraction

Considerable insight into phosphoinositide-regulated cytoplasmic functions has been gained by identifying phosphoinositide-effector proteins. Phosphoinositide-regulated nuclear functions however are fewer and less clear. To address this, we established a proteomic method based on neomycin extraction of intact nuclei to enrich for nuclear phosphoinositide-effector proteins. We identified 168 proteins harboring phosphoinositide-binding domains. Although the vast majority of these contained lysine/arginine-rich patches with the following motif, K/R-(X(n= 3-7)-K-X-K/R-K/R, we also identified a smaller subset of known phosphoinositide-binding proteins containing pleckstrin homology or plant homeodomain modules. Proteins with no prior history of phosphoinositide interaction were identified, some of which have functional roles in RNA splicing and processing and chromatin assembly. The remaining proteins represent potentially other novel nuclear phosphoinositide-effector proteins and as such strengthen our appreciation of phosphoinositide-regulated nuclear functions. DNA topology was exemplar among these: Biochemical assays validated our proteomic data supporting a direct interaction between phosphatidylinositol 4,5-bisphosphate and DNA Topoisomerase IIα. In addition, a subset of neomycin extracted proteins were further validated as phosphatidyl 4,5-bisphosphate-interacting proteins by quantitative lipid pull downs. In summary, data sets such as this serve as a resource for a global view of phosphoinositide-regulated nuclear functions.

Mammalian diacylglycerol kinases: molecular interactions and biological functions of selected isoforms

The mammalian diacylglycerol kinases (DGK) are a group of enzymes having important roles in regulating many biological processes. Both the product and the substrate of these enzymes, i.e. diacylglycerol and phosphatidic acid, are important lipid signalling molecules. Each DGK isoform appears to have a distinct biological function as a consequence of its location in the cell and/or the proteins with which it associates. This review discusses three of the more extensively studied forms of this enzyme, DGKalpha, DGKvarepsilon, and DGKzeta. DGKalpha has an important role in immune function and its activity is modulated by several mechanisms. DGKvarepsilon has several unique features among which is its specificity for arachionoyl-containing substrates, suggesting its importance in phosphatidylinositol cycling. DGKzeta is expressed in many tissues and also has several mechanisms to regulate its functions. It is localized in several subcellular organelles, including the nucleus. The current state of our understanding of the properties and functions of these proteins is reviewed.

IP3-dependent nuclear Ca2+ signalling in the mammalian heart

In cardiac myocytes the type-2 inositol 1,4,5-trisphosphate receptor (IP(3)R2) is the predominant isoform expressed. The IP(3)R2 channel is localized to the SR and to the nuclear envelope. We studied IP(3)-dependent nuclear Ca(2+) signals ([Ca(2+)](Nuc)) in permeabilized atrial myocytes and in isolated cardiac nuclei. In permeabilized myocytes IP(3) (20 microm) and the more potent IP(3)R agonist adenophostin (5 microm) caused an elevation of [Ca(2+)](Nuc). An IP(3)-dependent increase of [Ca(2+)](Nuc) was still observed after pretreatment with tetracaine to block Ca(2+) release from ryanodine receptors (RyRs), and the effect of IP(3) was partially reversed or prevented by the IP(3)R blockers heparin and 2-APB. Isolated nuclei were superfused with an internal solution containing the Ca(2+) indicator fluo-4 dextran. Exposure to IP(3) (10 microm) and adenophostin (0.5 microm) increased [Ca(2+)](Nuc) by 25 and 27%, respectively. [Ca(2+)](Nuc) increased to higher levels than [Ca(2+)](Cyt) immediately adjacent to the outer membrane of the nuclear envelope, suggesting that a significant portion of nuclear IP(3) receptors are facing the nucleoplasm. When nuclei were pretreated with heparin or 2-APB, IP(3) failed to increase [Ca(2+)](Nuc). Isolated nuclei were also loaded with the membrane-permeant low-affinity Ca(2+) probe fluo-5N AM which compartmentalized into the nuclear envelope. Exposure to IP(3) and adenophostin resulted in a decrease of the fluo-5N signal that could be prevented by heparin. Stimulation of IP(3)R caused depletion of the nuclear Ca(2+) stores by approximately 60% relative to the maximum depletion produced by the ionophores ionomycin and A23187. The fluo-5N fluorescence decrease was particularly pronounced in the nuclear periphery, suggesting that the nuclear envelope may represent the predominant nuclear Ca(2+) store. The data indicate that IP(3) can elicit Ca(2+) release from cardiac nuclei resulting in localized nuclear Ca(2+) signals.

The PKC pathway and in particular its β1 isoform is clearly involved in meiotic arrest maintenance but poorly in FSH-induced meiosis resumption of the mouse cumulus cell enclosed oocyte

PKC modulators were used to investigate the role of the PKC pathway either on the maintenance of meiotic arrest or on FSH-induced maturation of mouse cumulus cell enclosed oocytes (CEOs). (1) Whereas PKC activation (PMA 8 microM) overcomed clearly the HX-maintained meiotic arrest (83.7 +/- 3.6% vs. 16.1 +/- 10.6% GVBD oocytes), PKC inhibition (Calphostin C 100 nM) did not. On the contrary, it better maintained the meiotic arrest than HX alone. (2) No significant effect of PKC activation or inhibition was observed. (3) HX alone maintained PKCbeta1 in the cytoplasm, whereas FSH and PKC activation induced partly its translocation into the nucleus. The results show that whereas the PKC pathway is clearly involved in maintenance of the meiotic arrest through PKCbeta1, it is not involved in FSH-induced meiosis of CEOs.

Protein kinase C inhibits binding of diacylglycerol kinase-zeta to the retinoblastoma protein

We previously showed that the retinoblastoma protein (pRB), a key regulator of G1 to S-phase transition of the cell cycle, binds to and stimulates diacylglycerol kinase-zeta (DGKzeta) to phosphorylate the lipid second messenger diacylglycerol into phosphatidic acid. pRB binds to the MARCKS phosphorylation-site domain of DGKzeta that can be phosphorylated by protein kinase C (PKC). Here, we report that activation of PKC by phorbol ester inhibits DGKzeta binding to pRB. Ro 31-8220, a specific inhibitor of PKC, alleviated this inhibition of binding. Mimicking of PKC phosphorylation of serine residues (by S/D but not S/N mutations) within the DGKzeta-MARCKS phosphorylation-site domain also prevented DGKzeta binding to pRB, suggesting that PKC phosphorylation of these residues negatively regulates the interaction between DGKzeta and pRB. In PKC overexpression studies, it appeared that activation of particularly the (wild-type) PKCalpha isoform inhibits DGKzeta binding to pRB, whereas dominant-negative PKCalpha neutralized this inhibition. PKCalpha activation thus prevents DGKzeta regulation by pRB, which may have implications for nuclear diacylglycerol and phosphatidic acid levels during the cell cycle.

Disruption of diacylglycerol metabolism impairs the induction of T cell anergy

Anergic T cells have altered diacylglycerol metabolism, but whether that altered metabolism has a causative function in the induction of T cell anergy is not apparent. To test the importance of diacylglycerol metabolism in T cell anergy, we manipulated diacylglycerol kinases (DGKs), which are enzymes that terminate diacylglycerol-dependent signaling. Overexpression of DGK-alpha resulted in a defect in T cell receptor signaling that is characteristic of anergy. We generated DGK-alpha-deficient mice and found that DGK-alpha-deficient T cells had more diacylglycerol-dependent T cell receptor signaling. In vivo anergy induction was impaired in DGK-alpha-deficient mice. When stimulated in anergy-producing conditions, T cells lacking DGK-alpha or DGK-zeta proliferated and produced interleukin 2. Pharmacological inhibition of DGK-alpha activity in DGK-zeta-deficient T cells that received an anergizing stimulus proliferated similarly to wild-type T cells that received CD28 costimulation and prevented anergy induction. Our findings suggest that regulation of diacylglycerol metabolism is critical in determining whether activation or anergy ensues after T cell receptor stimulation.

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.

Mass spectrometry determination of endonuclear phospholipid composition and dynamics

Mammalian cell lipid analyses using tandem electrospray ionization mass spectrometry, in conjunction with stable isotope labeling, permit unparalleled access to membrane phospholipid molecular species compositions and turnover. Lipidomic data from isolable compartments of lipid second messenger generation, such as membrane-free nuclei, can provide dynamic insights into the topology of phospholipid turnover. For example, ESI-MS/MS precursor scans of characteristic phosphocholine m/z 184(+) fragments reveal a highly saturated endonuclear phosphatidylcholine pool with homeostatic maintenance properties. A spatially distinct CDPcholine pathway yields, within minutes of choline-d(9) labeling, unsaturated endonuclear phosphatidylcholines progressively remodeled to more saturated species evidenced by tracking the deuteriated headgroup through precursor scans of phosphocholine-d(9) (m/z 193(+) fragment). Among the other endonuclear phospholipids, diacyl phosphatidylethanolamines (neutral loss of m/z 141(+)) are also highly saturated compared with those of whole cell whereas, phophatidylinositols (precursor scans of m/z 241(-) fragment) are essentially identical in nuclei and whole cells. Moreover, the pattern of myo-inositol-d(6) acquisition into endonuclear phosphatidylinositol (precursor scans of m/z 247(-) fragment) is inconsistent with compartment-specific synthesis. Endonuclear sphingomyelins (seen in precursor scans of m/z 184(+) and confirmed from precursor scans of m/z 168(-) fragments) are enriched but similar in composition to whole cell species whereas endonuclear phosphatidylserines (neutral loss of m/z 87(-)) are more saturated than their whole cell counterparts. The focus of described methodologies emphasize their value in probing the compositions and dynamics of endonuclear phospholipids, but in principle may be extended to exploration of other isolable compartments including ER or plasma membranes.

Diacylglycerol, phosphatidic acid, and the converting enzyme, diacylglycerol kinase, in the nucleus

There exists phosphoinositide (PI) cycle in the nucleus, which is operated differentially from the classical PI cycle at the plasma membrane. Evidence has been accumulated that nuclear PIs and the related enzymes are closely involved in a variety of nuclear processes, although the details remain to be elucidated. In this mini review, some components of PI cycle, i.e., diacylglycerol, phosphatidic acid, and the converting enzyme, diacylglycerol kinase, in the nucleus are discussed with focusing on the lipid metabolism, cell cycle regulation, and animal models.

Nuclear inositide signalling -- expansion, structures and clarification

The extent and content of this review issue highlights how our understanding of lipid signalling in the nucleus has grown, both in what we actually know, and the breadth of signalling pathways that we now have to consider. Here, a few key issues with regard to nuclear inositide signalling are briefly addressed.

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.

Dynamic lipidomics of the nucleus

Once nuclear envelope membranes have been removed from isolated nuclei, around 6% of mammalian cell phospholipid is retained within the nuclear matrix, which calculations suggest may occupy 10% of the volume of this subcellular compartment. It is now acknowledged that endonuclear phospholipid, largely ignored for the past 40 years, provides substrate for lipid-mediated signaling events. However, given its abundance, it likely also has other as yet incompletely defined roles. Endonuclear phosphatidylcholine is the predominant phospholipid comprising distinct and highly saturated molecular species compared with that of the whole cell. Moreover, this unusual composition is subject to tight homeostatic maintenance even under conditions of extreme dietary manipulation, presumably reflecting a functional requirement for highly saturated endonuclear phosphatidylcholine. Recent application of new lipidomic technologies exploiting tandem electrospray ionization mass spectrometry in conjunction with deuterium stable isotope labeling have permitted us to probe not just molecular species compositions but endonuclear phospholipid acquisition and turnover with unparalleled sensitivity and specificity. What emerges is a picture of a dynamic pool of endonuclear phospholipid subject to autonomous regulation with respect to bulk cellular phospholipid metabolism. Compartmental biosynthesis de novo of endonuclear phosphatidylcholine contrasts with import of phosphatidylinositol synthesized elsewhere. However, irrespective of the precise temporal-spatial-dynamic relationships underpinning phospholipid acquisition, derangement of endonuclear lipid-mediated signaling from these parental phospholipids halts cell growth and division indicating a pivotal control point. This in turn defines the manipulation of compartmentalized endonuclear phospholipid acquisition and metabolism as intriguing potential targets for the development of future antiproliferative strategies.

Signaling roles of diacylglycerol kinases

Diacylglycerol kinases (DGKs) attenuate diacylglycerol signaling by converting this lipid to phosphatidic acid (PA). The nine mammalian DGKs that have been identified are widely expressed, but each isoform has a unique tissue and subcellular distribution. Their kinase activity is regulated by mechanisms that modify their access to diacylglycerol, directly affect their kinase activity, or alter their ability to bind to other proteins. In many cases, these enzymes regulate the activity of proteins that are modulated by either diacylglycerol or PA. Experiments using cultured cells and model organisms have demonstrated that DGKs have prominent roles in neuronal transmission, lymphocyte signaling, and carcinogenesis.

The retinoblastoma family proteins bind to and activate diacylglycerol kinase zeta

The retinoblastoma protein (pRB) is a tumor suppressor and key regulator of the cell cycle. We have previously shown that pRB interacts with phosphatidylinositol-4-phosphate 5-kinases, lipid kinases that can regulate phosphatidylinositol 4,5-bisphosphate levels in the nucleus. Here, we investigated pRB binding to another lipid kinase in the phosphoinositide cycle, diacylglycerol kinase (DGK) that phosphorylates the second messenger diacylglycerol to yield phosphatidic acid. We found that DGKzeta, but not DGKalpha or DGK, interacts with pRB in vitro and in vivo. Binding of DGKzeta to pRB is dependent on the phosphorylation status of pRB, since only hypophosphorylated pRB interacts with DGKzeta. DGKzeta also binds to the pRB-related pocket proteins p107 and p130 in vitro and in cells. Although DGKzeta did not affect the ability of pRB to regulate E2F-mediated transcription, we found that pRB, p107, and p130 potently stimulate DGKzeta activity in vitro. Finally, overexpression of DGKzeta in pRB-null fibroblasts reconstitutes a cell cycle arrest induced by gamma-irradiation. These results suggest that DGKzeta may act in vivo as a downstream effector of pRB to regulate nuclear levels of diacylglycerol and phosphatidic acid.

Integral membrane lipid phosphatases/phosphotransferases: common structure and diverse functions

Phospholipids and sphingolipids play critical roles in signal transduction, intracellular membrane trafficking, and control of cell growth and survival. We discuss recent progress in the identification and characterization of a family of integral membrane proteins with central roles in bioactive lipid metabolism and signalling. These five groups of homologous proteins, which we collectively term LPTs (lipid phosphatases/phosphotransferases), are characterized by a core domain containing six transmembrane-spanning alpha-helices connected by extramembrane loops, two of which interact to form the catalytic site. LPT family members are localized to all major membrane compartments of the cell. The transmembrane topology of these proteins places their active site facing the lumen of endomembrane compartments or the extracellular face of the plasma membrane. Sequence conservation between the active site of the LPPs (lipid phosphate phosphatases), SPPs (sphingosine phosphate phosphatases) and the recently identified SMSs (sphingomyelin synthases) with vanadium-dependent fungal oxidases provides a framework for understanding their common catalytic mechanism. LPPs hydrolyse LPA (lysophosphatidic acid), S1P (sphingosine 1-phosphate) and structurally-related substrates. Although LPPs can dephosphorylate intracellularly generated substrates to control intracellular lipid metabolism and signalling, their best understood function is to regulate cell surface receptor-mediated signalling by LPA and S1P by inactivating these lipids at the plasma membrane or in the extracellular space. SPPs are intracellularly localized S1P-selective phosphatases, with key roles in the pathways of sphingolipid metabolism linked to control of cell growth and survival. The SMS enzymes catalyse the interconversion of phosphatidylcholine and ceramide with sphingomyelin and diacylglycerol, suggesting a pivotal role in both housekeeping lipid synthesis and regulation of bioactive lipid mediators. The remaining members of the LPT family, the LPR/PRGs (lipid phosphatase-related proteins/plasticity-related genes) and CSS2s (type 2 candidate sphingomyelin synthases), are presently much less well studied. These two groups include proteins that lack critical amino acids within the catalytic site, and could therefore not use the conserved LPT reaction mechanism to catalyse lipid phosphatase or phosphotransferase reactions. In this review, we discuss recent ideas about their possible biological activities and functions, which appear to involve regulation of cellular morphology and, possibly, lipid metabolism and signalling in the nuclear envelope.

Functional endothelin ET B receptors are selectively expressed in human oligodendrogliomas

Endothelin-1 (ET-1), a vasoactive and mitogenic peptide mainly produced by vascular endothelial cells, may be involved in the progression of several human tumors. Here, we present an immunohistochemical analysis of the expression pattern of ET-1 receptor subtypes (ET(A)-R and ET(B)-R) and a functional study of their potential role in human oligodendrogliomas and oligoastrocytomas. By comparison, we assessed the corresponding expression patterns of glioblastomas. Interestingly, a nuclear localization of ET-1 receptor subtypes (associated or not with a cytoplasmic labeling) was constantly observed in tumor cells from all three glioma types. Moreover, we noted a distinct receptor distribution in the different gliomas: a nuclear expression of ET(B)-R by tumor cells was found to be restricted to oligodendrogliomas and oligoastrocytomas, while a nuclear expression of ET(A)-R was only detected in tumor cells from some glioblastomas. Using primary cultures of oligodendroglial tumor cells, we confirmed the selective expression of nuclear ET(B)-R, together with a plasma membrane expression, and further demonstrated that this receptor was functionally coupled to intracellular signaling pathways known to be involved in cell survival and/or proliferation: extracellular signal-regulated kinase and focal adhesion kinase activation, actin cytoskeleton reorganization. In addition, impairment of ET(B)-R activation in these cells by in vitro treatment with an ET(B)-R-specific antagonist induced cell death. These data point to ET-1 as a possible survival factor for oligodendrogliomas via ET(B)-R activation and suggest that ET(B)-R-specific antagonists might constitute a potential therapeutic alternative for oligodendrogliomas.

Nuclear phospholipase C-β1b activation during G2/M and late G1 phase in nocodazole-synchronized HL-60 cells

In this study, the activity of nuclear phosphatidylinositol-specific phosholipase C (PI-PLC) was investigated in HL-60 cells blocked at G(2)/M phase by the addition of nocodazole, and released into medium as synchronously progressing cells. Two peaks of an increase in the nuclear PI-PLC activities were detected; an early peak reached a maximum at 1 h after release from the nocodazole block, and a second increase was detected at 8.5 h after the release. Immunoprecipitation studies indicated that the increase in the activity was due to the activation of the nuclear PI-PLC-beta(1). Western blot analysis demonstrated no changes in the level of both a and b splicing variants of PI-PLC-beta(1) in the nuclei of cells isolated at either 1 h or 8.5 h after the block. However, an increase in the serine-phosphorylation of PI-PLC-beta(1b) was detected in the nuclei of HL-60 cells isolated at 1 and 8.5 h after the block, and the presence of MEK-inhibitor PD98059 completely inhibited both the serine phosphorylation and the increase in the PI-PLC activities in vitro. The presence of PI-PLC inhibitor prevented the progression of HL-60 cells through the G(1) into S phase of the cell cycle. These results demonstrate that two peaks of nuclear PI-PLC activities, which are due to a PD98059-sensitive phosphorylation of nuclear PLC-beta(1b) on serine, occur at the G(2)/M and late G(1) phase and are necessary for the progression of the cells through the cell cycle.

Membrane fluidity is a key modulator of membrane binding, insertion, and activity of 5-lipoxygenase

Mammalian 5-lipoxygenase (5-LO) catalyzes conversion of arachidonic acid to leukotrienes, potent mediators of inflammation and allergy. Upon cell stimulation, 5-LO selectively binds to nuclear membranes and becomes activated, yet the mechanism of recruitment of 5-LO to nuclear membranes and the mode of 5-LO-membrane interactions are poorly understood. Here we show that membrane fluidity is an important determinant of membrane binding strength of 5-LO, penetration into the membrane hydrophobic core, and activity of the enzyme. The membrane binding strength and activity of 5-LO increase with the degree of lipid acyl chain cis-unsaturation and reach a plateau with 1-palmitoyl-2-arachidonolyl-sn-glycero-3-phosphocholine (PAPC). A fraction of tryptophans of 5-LO penetrate into the hydrocarbon region of fluid PAPC membranes, but not into solid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine membranes. Our data lead to a novel concept of membrane binding and activation of 5-LO, suggesting that arachidonic-acid-containing lipids, which are present in nuclear membranes at higher fractions than in other cellular membranes, may facilitate preferential membrane binding and insertion of 5-LO through increased membrane fluidity and may thereby modulate the activity of the enzyme. The data presented in this article and earlier data allow construction of a model for membrane-bound 5-LO, including the angular orientation and membrane insertion of the protein.

Distribution of C-terminal splice variant of G alpha i2 in rat and monkey brain

The significance of Galphai2 in neural signal transmission is well defined. However, the function of its alternative splice variant named sGi2 is unknown. Therefore here, we have studied the localization of sGi2 protein in rat and monkey brain at light and electron microscopy level. We found that this novel protein is widely expressed in rat and monkey brain regions, which are known to play crucial role in brain functions. Hippocampus, cerebral cortex, amygdala, thalamus, striatum, nucleus accumbens, olfactory tubercle and dopaminergic cell groups of substantia nigra, hypothalamus and olfactory bulb showed strong labeling with anti-sGi2. At subcellular level, sGi2 protein was localized in intracellular compartments, including endoplasmic reticulum, Golgi complex, mitochondria and nucleus. This protein was also found localized extra-synaptically in both axons and spines, which were making excitatory as well as inhibitory synaptic contacts. Moreover, the frequent localization of sGi2 protein in neck of spines further suggests that this protein may not engage directly in neuronal signal transmission but could influence other participating proteins of this process.

Phosphatidylcholine-specific phospholipase C in mitogen-stimulated fibroblasts

To investigate expression, subcellular localization and mechanisms of translocation of phosphatidylcholine-specific phospholipase C (PC-PLC) during the cell proliferative response, biochemical, immunoblotting, and immunofluorescence analyses were performed on quiescent and mitogen-stimulated NIH-3T3 fibroblasts. Platelet-derived growth factor (PDGF), insulin and 12-O-tetradecanoylphorbol-13-acetate induced, in 10-60 min, PC-PLC translocation from a perinuclear cytoplasmic area to the plasma membrane. Following cell exposure to PDGF (60 min), the overall PC-PLC expression increased up to 2-3x, while the enzyme activity increased 5x in total cell lysates, 2x in the plasma membrane, and 4x in the nucleus; moreover, confocal laser scanning microscopy showed a progressive externalization of PC-PLC on the outer plasma membrane surface and its accumulation in the nuclear matrix. Pre-incubation of cells with the PC-PLC inhibitor tricyclodecan-9-yl potassium xanthate (D609), before PDGF-stimulation, not only reduced the enzyme activity in total cell lysates as well as in plasma membrane and nuclear fractions, but also blocked the mechanisms of PC-PLC subcellular redistribution. These effects were associated with a D609-induced long-lasting cell cycle block in Go.

Nuclear phosphoinositides and their functions

Phosphoinositides are minor components of biological membranes, which have emerged as essential regulators of a variety of cellular processes, both on the plasma membrane and on several intracellular organelles. The versatility of these lipids stems from their ability to function either as substrates for the generation of second messengers, as membrane-anchoring sites for cytosolic proteins or as regulators of the actin cytoskeleton. Despite a vast literature demonstrating the presence of phosphoinositides in the nucleus, only recently has the function(s) of the nuclear pool of these lipids and their soluble analogues, inositol polyphosphates, started to emerge. These compounds have been shown to serve as essential co-factors for several nuclear processes, including DNA repair, transcription regulation and RNA dynamics. In this light, phosphoinositides and inositol polyphosphates might represent high turnover activity switches for nuclear complexes responsible for these processes. The regulation of these large machineries would be linked to the phosphorylation state of the inositol ring and limited temporally and spatially based on the synthesis and degradation of these molecules.

Functional endothelin receptors are present on nuclei in cardiac ventricular myocytes

Endothelins are thought to act through two specific, plasmalemmal G protein-coupled receptor subtypes, ETAR and ETBR. However, in subfractionated cardiac membranes, ETAR immunoreactivity was detected only in the plasma membrane whereas ETBR immunoreactivity was detected predominantly in membranes of intracellular origin. Confocal microscopy demonstrated the presence of intracellular ETAR and ETBR in ventricular myocytes. ETAR were primarily on plasma membrane (surface membranes and transverse-tubules) and to a lesser extent on the nucleus while ETBR localized primarily to the nuclei. Western blot analysis of nuclei isolated from the heart indicated the presence of endothelin receptors: both ETAR and ETBR copurified with nucleoporin 62, whereas markers of endoplasmic reticulum and Golgi membranes were depleted. Radioligand binding studies revealed that isolated nuclei contain specific [125I]ET-1 binding sites. Specific [125I]ET-1 binding was reduced by 70-80% using the ETAR-selective antagonist BQ610 and 20-30% using the ETBR-specific antagonist BQ788. IRL-1620, an ETBR-specific agonist, also reduced [125I]ET-1 binding. Furthermore, ET-1 and IRL-1620 altered the incorporation of 32P into nuclear proteins and caused a transient increase in nuclear Ca2+ concentration. Hence, cardiac nuclei possess both ETAR and ETBR subtypes, which are functional with respect to ligand binding and are coupled to signaling mechanisms within the nuclear membrane.

Protein kinase C nu/protein kinase D3 nuclear localization, catalytic activation, and intracellular redistribution in response to G protein-coupled receptor agonists

The protein kinase D (PKD) family consists of three serine/threonine kinases: PKC micro/PKD, PKD2, and PKCnu/PKD3. Whereas PKD has been the focus of most studies, virtually nothing is known about the effect of G protein-coupled receptor agonists (GPCR) on the regulatory properties and intracellular distribution of PKD3. Consequently, we examined the mechanism that mediates its activation and intracellular distribution. GPCR agonists induced a rapid activation of PKD3 by a protein kinase C (PKC)-dependent pathway that leads to the phosphorylation of the activation loop of PKD3. Comparison of the steady-state distribution of endogenous or tagged PKD3 versus PKD and PKD2 in unstimulated cells indicated that whereas PKD and PKD2 are predominantly cytoplasmic, PKD3 is present both in the nucleus and cytoplasm. This distribution of PKD3 results from its continuous shuttling between both compartments by a mechanism that requires a nuclear import receptor and a competent CRM1-nuclear export pathway. Cell stimulation with the GPCR agonist neurotensin induced a rapid and reversible plasma membrane translocation of PKD3 that is PKC-dependent. Interestingly, the nuclear accumulation of PKD3 can be dramatically enhanced in response to its activation. Thus, this study demonstrates that the intracellular distribution of PKD isoenzymes are distinct, and suggests that their signaling properties are regulated by differential localization.

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.

Association of diacylglycerol kinase zeta with protein kinase C alpha: spatial regulation of diacylglycerol signaling

Activation of PKC depends on the availability of DAG, a signaling lipid that is tightly and dynamically regulated. DAG kinase (DGK) terminates DAG signaling by converting it to phosphatidic acid. Here, we demonstrate that DGKzeta inhibits PKCalpha activity and that DGK activity is required for this inhibition. We also show that DGKzeta directly interacts with PKCalpha in a signaling complex and that the binding site in DGKzeta is located within the catalytic domain. Because PKCalpha can phosphorylate the myristoylated alanine-rich C-kinase substrate (MARCKS) motif of DGKzeta, we tested whether this modification could affect their interaction. Phosphorylation of this motif significantly attenuated coimmunoprecipitation of DGKzeta and PKCalpha and abolished their colocalization in cells, indicating that it negatively regulates binding. Expression of a phosphorylation-mimicking DGKzeta mutant that was unable to bind PKCalpha did not inhibit PKCalpha activity. Together, our results suggest that DGKzeta spatially regulates PKCalpha activity by attenuating local accumulation of signaling DAG. This regulation is impaired by PKCalpha-mediated DGKzeta phosphorylation.

Nuclear lipids: new functions for old molecules?

It is becoming increasingly evident that stimulation of nuclear lipid metabolism plays a central role in many signal transduction pathways that ultimately result in various cell responses including proliferation and differentiation. Nuclear lipid metabolism seems to be at least as complex as that existing at the plasma membrane. However, a distinctive feature of nuclear lipid biochemical pathways is their operational independence from their cell periphery counterparts. Although initially it was thought that nuclear lipids would serve as a source for second messengers, recent evidence points to the likelihood that lipids present in the nucleus also fulfil other roles. The aim of this review is to highlight the most intriguing advances made in the field over the last year, such as the production of new probes for the in situ mapping of nuclear phosphoinositides, the identification of two sources for nuclear diacylglycerol production, the emerging details about the peculiar regulation of nuclear phosphoinositide synthesizing enzymes, and the distinct possibility that nuclear lipids are involved in processes such as chromatin organization and pre-mRNA splicing.

MgATP may depress de novo neuronal nuclear PAF generation by promoting the formation of alkylacylglycerophosphate, an inhibitor of alkylglycerophosphate acetyltransferase

MgATP substantially inhibited 1-alkyl-sn-glycero-3-phosphate (AGP) acetyltransferase found in neuronal nuclei. Other nucleotides and the ATP analogue AMP-PNP did not show a comparable inhibition. MgATP inhibition decreased in the presence of bovine serum albumin or the fatty acyl CoA synthetase inhibitor, Triacsin C. MgATP inhibition increased when nuclei were preincubated in 50 mM Tris-HCl (pH 7.4)/1 mM MgCl(2) at 37 degrees C, and preincubations elevated levels of nuclear free fatty acid. Exogenous free fatty acid, added to the acetylation incubations, increased the inhibition seen in the presence of MgATP. Oleoyl CoA, in the absence of MgATP, also inhibited AGP acetylation. These results suggested that MgATP supported the conversion of nuclear free fatty acids to fatty acyl CoA. Fatty acyl CoA may directly inhibit nuclear AGP acetyltransferase, but inhibition brought about by MgATP was competitive for the AGP substrate, suggesting an inhibitor close in structure to AGP. 1-Hexadecyl-2-arachidonoyl-sn-glycero-3-phosphate was identified as a competitive inhibitor for AGP in the acetylation reaction. Neuronal nuclei can convert AGP to 1-alkyl-2-acyl-sn-glycero-3-phosphate (AAcylGP), a reaction dependent upon MgATP and the presence of acetyl CoA or free CoA. This nuclear acylation was increased by free fatty acid addition and was seen using oleoyl CoA in the absence of MgATP. Nuclear AAcylGP formation was inhibited by bovine serum albumin and by Triacsin C. Thus, nuclear AGP acetyltransferase may be regulated by AGP acyltransferase activity and the availability of MgATP, a nucleotide that is rapidly lost during brain ischemia.

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.

Carboxyl-terminal basic amino acids in the X domain are essential for the nuclear import of phospholipase C δ1

BACKGROUND

Although phospholipase C (PLC)delta1 containing a functional nuclear export signal (NES) is normally localized at the plasma membrane and in the cytoplasm, it shuttles between the nucleus and the cytoplasm. Since nucleocytoplasmic shuttling of a molecule is generally regulated by a balance between its NES and the nuclear localization signal (NLS), we examined whether PLCdelta1 contains an NLS sequence.

RESULTS

A region corresponding to the C terminus of the X domain and the XY-linker, which contains clusters of basic amino acid residues, was essential for the nuclear import of PLCdelta1 in Madin-Darby canine kidney cells. A series of point mutations on lysine residues in this region revealed that K432 and K434 in combination were important for the nuclear import. A short synthetic peptide corresponding to residues 429-442, however, was not able to function as an NLS sequence when they were injected into the cytoplasm in a carrier-conjugated form. Neither a longer peptide equivalent to PLCdelta1 412-498 fused to a protein tag consisting of glutathione S-transferase and green fluorescent protein was imported to the nucleus after microinjection into the cytoplasm.

CONCLUSION

The nuclear import of PLCdelta1 requires the C-terminus of the X domain, particularly the amino acid residues K432 and K434, and the XY-linker. The region alone, however, cannot serve as a functional NLS. The machinery for nuclear transport may require additional structural component(s) of the enzyme.

Specific natural DNA-bound lipids in post-genome era. The lipid conception of chromatin organization

Two pools of DNA-bound lipids were isolated from DNA supramolecular complex (SC-DNA): loosely bound (extracted with 35% ethanol) and tightly bound lipids (extracted after additional treatment DNase I). The compositions of the two lipid pools from different sources (rat thymus, liver, loach sperm, pigeon erythrocytes, Zajdel ascites hepatoma, Ehrlich ascites carcinoma, sarcoma 37, Escherichia coli B and T2 phage) were studied. The possible functions of DNA-bound lipids, especially of cardiolipin and cholesterol, at the attachment of DNA loops to the nuclear matrix, in DNA replicon organization, replication and transcription are discussed.

Type I PIPkinases interact with and are regulated by the retinoblastoma susceptibility gene product-pRB

Inositide signaling at the plasma membrane has been implicated in the regulation of numerous cellular processes including cytoskeletal dynamics, vesicle trafficking, and gene transcription. Studies have also shown that a distinct inositide pathway exists in nuclei, where it may regulate nuclear processes such as mRNA export, cell cycle progression, gene transcription, and DNA repair. We previously demonstrated that nuclear PtdIns(4,5)P(2) synthesis is stimulated during progression from G1 through S phase, although mechanistic details of how cell cycle progression impinges on the regulation of nuclear inositides is unknown. In this study, we demonstrate that pRB, which regulates progression of cells from G1 through S phase interacts both in vitro and in vivo with Type I PIPkinases, the enzymes responsible for nuclear PtdIns(4,5)P(2) synthesis. Moreover, this interaction stimulates the activity of Type Ialpha PIPkinase in an in vitro assay. Using murine erythroleukamia (MEL) cells expressing a temperature-sensitive mutant of large T antigen (LTA), we demonstrate changes in vivo in nuclear PtdIns(4,5)P(2) levels that are consistent with the ability of LTA to disrupt pRB/Type I interactions. This study, for the first time, provides a potential mechanism for how cell cycle progression could regulate the levels of nuclear inositides.

Proliferating or differentiating stimuli act on different lipid-dependent signaling pathways in nuclei of human leukemia cells

Previous results have shown that the human promyelocytic leukemia HL-60 cell line responds to either proliferating or differentiating stimuli. When these cells are induced to proliferate, protein kinase C (PKC)-beta II migrates toward the nucleus, whereas when they are exposed to differentiating agents, there is a nuclear translocation of the alpha isoform of PKC. As a step toward the elucidation of the early intranuclear events that regulate the proliferation or the differentiation process, we show that in the HL-60 cells, a proliferating stimulus (i.e., insulin-like growth factor-I [IGF-I]) increased nuclear diacylglycerol (DAG) production derived from phosphatidylinositol (4,5) bisphosphate, as indicated by the inhibition exerted by 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine and U-73122 (1-[6((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione), which are pharmacological inhibitors of phosphoinositide-specific phospholipase C. In contrast, when HL-60 cells were induced to differentiate along the granulocytic lineage by dimethyl sulfoxide, we observed a rise in the nuclear DAG mass, which was sensitive to either neomycin or propranolol, two compounds with inhibitory effect on phospholipase D (PLD)-mediated DAG generation. In nuclei of dimethyl sulfoxide-treated HL-60 cells, we observed a rise in the amount of a 90-kDa PLD, distinct from PLD1 or PLD2. When a phosphatidylinositol (4,5) bisphosphate-derived DAG pool was generated in the nucleus, a selective translocation of PKC-beta II occurred. On the other hand, nuclear DAG derived through PLD, recruited PKC-alpha to the nucleus. Both of these PKC isoforms were phosphorylated on serine residues. These results provide support for the proposal that in the HL-60 cell nucleus there are two independently regulated sources of DAG, both of which are capable of acting as the driving force that attracts to this organelle distinct, DAG-dependent PKC isozymes. Our results assume a particular significance in light of the proposed use of pharmacological inhibitors of PKC-dependent biochemical pathways for the therapy of cancer disease.

Generation of diacylglycerol molecular species through the cell cycle: a role for 1-stearoyl, 2-arachidonyl glycerol in the activation of nuclear protein kinase C-βII at G2/M

Protein kinase C (PKC) is a family of 11 isoenzymes that are differentially involved in the regulation of cell proliferation. PKC-βII, a mitotic lamin kinase, has been shown previously to translocate to the nucleus at G2/M and this was coupled to the generation of nuclear diacylglycerol. However, it is not clear how isoenzyme selective translocation and nuclear targeting is achieved during cell cycle. To investigate further the role of nuclear diacylglycerol we measured PKC isoenzyme translocation and analysed diacylglycerol species at different stages of the cell cycle in U937 cells synchronized by centrifugal elutriation. Translocation of PKC-βII to the membrane fraction, an indicator of activation, occurred at S and G2/M, although PKC-βII was targeted to the nucleus only at G2/M. Levels of nuclear diacylglycerol, specifically tetraunsaturated species, increased during G2/M. By contrast, there were no obvious changes in nuclear phosphatidic acid species or mass. 1-stearoyl, 2-arachidonyl glycerol (SAG), the major polyunsaturated nuclear diacylglycerol, was able to activate classical PKC isoenzymes (PKC-α andβ), but was less effective for activation of novel isoenzymes(PKC-δ), in an in vitro PKC assay. We propose that PKC-βII nuclear translocation during G2/M phase transition is mediated in part by generation of SAG at the nucleus.