Nuclear phospholipase C and signaling

Nuclear patterns of phosphatidylinositol 4,5- and 3,4-bisphosphate revealed by super-resolution microscopy differ between the consecutive stages of RNA polymerase II transcription

Phosphatidylinositol phosphates are powerful signaling molecules that orchestrate signaling and direct membrane trafficking in the cytosol. Interestingly, phosphatidylinositol phosphates also localize within the membrane-less compartments of the cell nucleus, where they participate in the regulation of gene expression. Nevertheless, current models of gene expression, which include condensates of proteins and nucleic acids, do not include nuclear phosphatidylinositol phosphates. This gap is partly a result of the missing detailed analysis of the subnuclear distribution of phosphatidylinositol phosphates and their relationships with gene expression. Here, we used quantitative dual-color direct stochastic optical reconstruction microscopy to analyze the nanoscale co-patterning between RNA polymerase II transcription initiation and elongation markers with respect to phosphatidylinositol 4,5- or 3,4-bisphosphate in the nucleoplasm and nuclear speckles and compared it with randomized data and cells with inhibited transcription. We found specific co-patterning of the transcription initiation marker P-S5 with phosphatidylinositol 4,5-bisphosphate in the nucleoplasm and with phosphatidylinositol 3,4-bisphosphate at the periphery of nuclear speckles. We showed the specific accumulation of the transcription elongation marker PS-2 and of nascent RNA in the proximity of phosphatidylinositol 3,4-bisphosphate associated with nuclear speckles. Taken together, this shows that the distinct spatial associations between the consecutive stages of RNA polymerase II transcription and nuclear phosphatidylinositol phosphates exhibit specificity within the gene expression compartments. Thus, in analogy to the cellular membranes, where phospholipid composition orchestrates signaling pathways and directs membrane trafficking, we propose a model in which the phospholipid identity of gene expression compartments orchestrates RNA polymerase II transcription.

Stimulation of phospholipase Cβ1 by Gαq promotes the assembly of stress granule proteins

[Figure: see text].

Modulation of DNA Damage Response by Sphingolipid Signaling: An Interplay that Shapes Cell Fate

Although once considered as structural components of eukaryotic biological membranes, research in the past few decades hints at a major role of bioactive sphingolipids in mediating an array of physiological processes including cell survival, proliferation, inflammation, senescence, and death. A large body of evidence points to a fundamental role for the sphingolipid metabolic pathway in modulating the DNA damage response (DDR). The interplay between these two elements of cell signaling determines cell fate when cells are exposed to metabolic stress or ionizing radiation among other genotoxic agents. In this review, we aim to dissect the mediators of the DDR and how these interact with the different sphingolipid metabolites to mount various cellular responses.

Immune Responses Alter Taste Perceptions: Immunomodulatory Drugs Shape Taste Signals during Treatments

Considering that nutrients are required in health and diseases, the detection and ingestion of food to meet the requirements is attributable to the sense of taste. Altered taste sensations lead to a decreased appetite, which is usually one of the frequent causes of malnutrition in patients with diseases. Ongoing taste research has identified a variety of drug pathways that cause changes in taste perceptions in cancer, increasing our understanding of taste disturbances attributable to aberrant mechanisms of taste sensation. The evidence discussed in this review, which addresses the implications of innate immune responses in the modulation of taste functions, focuses on the adverse effects on taste transmission from taste buds by immune modulators responsible for alterations in the perceived intensity of some taste modalities. Another factor, damage to taste progenitor cells that directly results in local effects on taste buds, must also be considered in relation to taste disturbances in patients with cancer. Recent discoveries discussed have provided new insights into the pathophysiology of taste dysfunctions associated with the specific treatments. SIGNIFICANCE STATEMENT: The paradigm that taste signals transmitted to the brain are determined only by tastant-mediated activation via taste receptors has been challenged by the immune modification of taste transmission through drugs during the processing of gustatory information in taste buds. This article reports the findings in a model system (mouse taste buds) that explain the basis for the taste dysfunctions in patients with cancer that has long been observed but never understood.

The role of phospholipase Cβ on the plasma membrane and in the cytosol: How modular domains enable novel functions

Phospholipase Cβ (PLCβ) is a signaling enzyme activated by G proteins to generate calcium signals. The catalytic core of PLCβ is surrounded by modular domains that mediate the interaction of the enzyme with known protein partners on the plasma membrane. The C-terminal region PLCβ contains a novel coiled-coil domain that is required for Gαq binding and activation. Recent work has shown that this domain also binds a number of cytosolic proteins that regulate protein translation, and that these proteins compete with Gαq for PLCβ binding. The ability of PLCβ to shuttle between the cytosol to impact protein translation and the plasma membrane to mediate calcium signals puts PLCβ in a central role in cell function.

OSBP-related protein 4L promotes phospholipase Cβ3 translocation from the nucleus to the plasma membrane in Jurkat T-cells

Phosphoinositide phospholipases C (PLCs) are a family of eukaryotic intracellular enzymes with important roles in signal transduction. In addition to their location at the plasma membrane, PLCs also exist within the cell nucleus where they are stored. We previously demonstrated that OSBP-related protein 4L (ORP4L) anchors cluster of differentiation 3ϵ (CD3ϵ) to the heterotrimeric G protein subunit (Gα_q/11) to control PLCβ3 relocation and activation. However, the underlying mechanism by which ORP4L facilitates PLCβ3 translocation remains unknown. Here, using confocal immunofluorescence microscopy and coimmunoprecipitation assays, we report that ORP4L stimulates PLCβ3 translocation from the nucleus to the plasma membrane in Jurkat T-cells in two steps. First, we found that ORP4L is required for the activation of Ras-related nuclear protein (RAN), a GTP-binding nuclear protein that binds to exportin 1 and eventually promotes the nuclear export of PLCβ3. Second, we also observed that ORP4L interacts with vesicle-associated membrane protein-associated protein A (VAPA) through its two phenylalanines in an acidic tract (FFAT) motif. This complex enabled PLCβ3 movement to the plasma membrane, indicating that PLCβ3 translocation occurs in a VAPA-dependent manner. This study reveals detailed mechanistic insight into the role of ORP4L in PLCβ3 redistribution from storage within the nucleus to the plasma membrane via RAN activation and interaction with VAPA in Jurkat T-cells.

The effect of imiquimod on taste bud calcium transients and transmitter secretion

BACKGROUND AND PURPOSE

Imiquimod is an immunomodulator approved for the treatment of basal cell carcinoma and has adverse side effects, including taste disturbances. Paracrine transmission, representing cell-cell communication within taste buds, has the potential to shape the final signals that taste buds transmit to the brain. Here, we tested the underlying assumption that imiquimod modifies taste transmitter secretion in taste buds of mice.

EXPERIMENTAL APPROACH

Taste buds were isolated from C57BL/6J mice. The effects of imiquimod on transmitter release in taste buds were measured using calcium imaging with cellular biosensors, and examining the net effect of imiquimod on taste-evoked ATP secretion from mouse taste buds.

KEY RESULTS

Up to 72% of presynaptic (Type III) taste cells responded to 100 μM imiquimod with an increase in intracellular Ca²⁺ concentrations. These Ca²⁺ responses were inhibited by thapsigargin, an inhibitor of the sarco/endoplasmic reticulum Ca²⁺ -ATPase, and by U73122, a PLC inhibitor, suggesting that the Ca²⁺ mobilization elicited by imiquimod was dependent on release from internal Ca²⁺ stores. Moreover, combining studies of Ca²⁺ imaging with cellular biosensors showed that imiquimod evoked secretion of 5-HT, which then provided negative feedback onto receptor (Type II) cells to reduce taste-evoked ATP secretion.

CONCLUSION AND IMPLICATIONS

Our results provide evidence that there is a subset of taste cells equipped with a range of intracellular mechanisms that respond to imiquimod. The findings are also consistent with a role of imiquimod as an immune response modifier, which shapes peripheral taste responses via 5-HT signalling.

BMP-2 Induced Expression of PLCβ1 That is a Positive Regulator of Osteoblast Differentiation

Bone morphogenetic protein 2 (BMP-2) is a critical growth factor that directs osteoblast differentiation and bone formation. Phosphoinositide-phospholipase Cβ 1 (PLCβ1) plays a crucial role in the initiation of the genetic program responsible for muscle differentiation. Differentiation of C2C12 mouse myoblasts in response to insulin stimulation is characterized by a marked increase in nuclear PLCβ1. Here, the function of PLCβ1 in the osteogenic differentiation was investigated. Briefly, in C2C12 cells treated with BMP-2 we assist to a remarkable increase in PLCβ1 protein and mRNA expression. The data regarding the influence on differentiation demonstrated that PLCβ1 promotes osteogenic differentiation by up-regulating alkaline phosphatase (ALP). Moreover, PLCβ1 is present in the nuclear compartment of these cells and overexpression of a cytosolic-PLCβ1mutant (cyt-PLCβ1), which lacks a nuclear localization sequence, prevented the differentiation of C2C12 cells into osteocytes. Recent evidence indicates that miRNAs act as important post transcriptional regulators in a large number of processes, including osteoblast differentiation. Since miR-214 is a regulator of Osterix (Osx) which is an osteoblast-specific transcription factor that is needful for osteoblast differentiation and bone formation, we further investigated whether PLCβ1 could be a potential target of miR-214 in the control of osteogenic differentiation by gain- and loss- of function experiment. The results indicated that inhibition of miR-214 in C2C12 cells significantly enhances the protein level of PLCβ1 and promotes C2C12 BMP-2-induced osteogenesis by targeting PLCβ1.

An integrated mechanism of cardiomyocyte nuclear Ca(2+) signaling

In cardiomyocytes, Ca(2+) plays a central role in governing both contraction and signaling events that regulate gene expression. Current evidence indicates that discrimination between these two critical functions is achieved by segregating Ca(2+) within subcellular microdomains: transcription is regulated by Ca(2+) release within nuclear microdomains, and excitation-contraction coupling is regulated by cytosolic Ca(2+). Accordingly, a variety of agonists that control cardiomyocyte gene expression, such as endothelin-1, angiotensin-II or insulin-like growth factor-1, share the feature of triggering nuclear Ca(2+) signals. However, signaling pathways coupling surface receptor activation to nuclear Ca(2+) release, and the phenotypic responses to such signals, differ between agonists. According to earlier hypotheses, the selective control of nuclear Ca(2+) signals by activation of plasma membrane receptors relies on the strategic localization of inositol trisphosphate receptors at the nuclear envelope. There, they mediate Ca(2+) release from perinuclear Ca(2+) stores upon binding of inositol trisphosphate generated in the cytosol, which diffuses into the nucleus. More recently, identification of such receptors at nuclear membranes or perinuclear sarcolemmal invaginations has uncovered novel mechanisms whereby agonists control nuclear Ca(2+) release. In this review, we discuss mechanisms for the selective control of nuclear Ca(2+) signals with special focus on emerging models of agonist receptor activation.

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.

K562 cell proliferation is modulated by PLCβ1 through a PKCα-mediated pathway

Phospholipase C β1 (PLCβ1) is known to play an important role in cell proliferation. Previous studies reported an involvement of PLCβ1 in G 0-G 1/S transition and G 2/M progression in Friend murine erythroleukemia cells (FELC). However, little has been found about its role in human models. Here, we used K562 cell line as human homologous of FELC in order to investigate the possible key regulatory role of PLCβ1 during cell proliferation of this human cell line. Our studies on the effects of the overexpression of both these isoforms showed a specific and positive connection between cyclin D3 and PLCβ1 in K562 cells, which led to a prolonged S phase of the cell cycle and a delay in cell proliferation. In order to shed light on this mechanism, we decided to study the possible involvement of protein kinases C (PKC), known to be direct targets of PLC signaling and important regulators of cell proliferation. Our data showed a peculiar decrease of PKCα levels in cells overexpressing PLCβ1. Moreover, when we silenced PKCα, by RNAi technique, in order to mimic the effects of PLCβ1, we caused the same upregulation of cyclin D3 levels and the same decrease of cell proliferation found in PLCβ1-overexpressing cells. The key features emerging from our studies in K562 cells is that PLCβ1 targets cyclin D3, likely through a PKCα-mediated-pathway, and that, as a downstream effect of its activity, K562 cells undergo an accumulation in the S phase of the cell cycle.

Phospholipase Cβ1 is linked to RNA interference of specific genes through translin-associated factor X

Phospholipase Cβ1 (PLCβ1) is a G-protein-regulated enzyme whose activity results in proliferative and mitogenic changes in the cell. We have previously found that in solution PLCβ1 binds to the RNA processing protein translin-associated factor X (TRAX) with nanomolar affinity and that this binding competes with G proteins. Here, we show that endogenous PLCβ1 and TRAX interact in SK-N-SH cells and also in HEK293 cells induced to overexpress PLCβ1. In HEK293 cells, TRAX overexpression ablates Ca(2+) signals generated by G protein-PLCβ1 activation. TRAX plays a key role in down-regulation of proteins by small, interfering RNA, and PLCβ1 overexpression completely reverses the 2- to 4-fold down-regulation of GAPDH by siRNA in HEK293 and HeLa cells as seen by an ∼4-fold recovery in both the transcript and protein levels. Also, down-regulation of endogenous PLCβ1 in HEK293 and HeLa cells allows for an ∼20% increase in siRNA(GAPDH) silencing. While PLCβ1 overexpression results in a 50% reversal of cell death caused by siRNA(LDH), it does not affect cell survival or silencing of other genes (e.g., cyclophilin, Hsp90, translin). PLCβ1 overexpression in HEK293 and HeLa cells causes a 30% reduction in the total amount of small RNAs. LDH and GAPDH are part of a complex that promotes H2B synthesis that allows cells to progress through the S phase. We find that PLCβ1 reverses the cell death and completely rescues H2B levels caused by siRNA knockdown of LDH or GAPDH. Taken together, our study shows a novel role of PLCβ1 in gene regulation through TRAX association.

Cholesterol: recapitulation of its active role during liver regeneration

Liver regeneration is a compensatory hyperplasia produced by several stimuli that promotes proliferation in order to provide recovery of the liver mass and architecture. This process involves complex signalling cascades that receive feedback from autocrine and paracrine pathways, recognized by parenchymal as well as non-parenchymal cells. Nowadays the dynamic role of lipids in biological processes is widely recognized; however, a systematic analysis of their importance during liver regeneration is still missing. Therefore, in this review we address the role of lipids including the bioactive ones such as sphingolipids, but with special emphasis on cholesterol. Cholesterol is not only considered as a structural component but also as a relevant lipid involved in the control of the intermediate metabolism of different liver cell types such as hepatocytes, hepatic stellate cells and Kupffer cells. Cholesterol plays a significant role at the level of specific membrane domains, as well as modulating the expression of sterol-dependent proteins. Moreover, several enzymes related to the catabolism of cholesterol and whose activity is down regulated are related to the protection of liver tissue from toxicity during the process of regeneration. This review puts in perspective the necessity to study and understand the basic mechanisms involving lipids during the process of liver regeneration. On the other hand, the knowledge acquired in this area in the past years, can be considered invaluable in order to provide further insights into processes such as general organogenesis and several liver-related pathologies, including steatosis and fibrosis.

Nuclear sphingolipid metabolism

Nuclear lipid metabolism is implicated in various processes, including transcription, splicing, and DNA repair. Sphingolipids play roles in numerous cellular functions, and an emerging body of literature has identified roles for these lipid mediators in distinct nuclear processes. Different sphingolipid species are localized in various subnuclear domains, including chromatin, the nuclear matrix, and the nuclear envelope, where sphingolipids exert specific regulatory and structural functions. Sphingomyelin, the most abundant nuclear sphingolipid, plays both structural and regulatory roles in chromatin assembly and dynamics in addition to being an integral component of the nuclear matrix. Sphingosine-1-phosphate modulates histone acetylation, sphingosine is a ligand for steroidogenic factor 1, and nuclear accumulation of ceramide has been implicated in apoptosis. Finally, nuclear membrane-associated ganglioside GM1 plays a pivotal role in Ca(2+) homeostasis. This review highlights research on the factors that control nuclear sphingolipid metabolism and summarizes the roles of these lipids in various nuclear processes.

Expression of phosphoinositide-specific phospholipase C isoenzymes in cultured astrocytes activated after stimulation with lipopolysaccharide

Signal transduction pathways, involved in cell cycle and activities, depend on various components including lipid signalling molecules, such as phosphoinositides and related enzymes. Many evidences support the hypothesis that inositol lipid cycle is involved in astrocytes activation during neurodegeneration. Previous studies investigated the pattern of expression of phosphoinositide-specific phospholipase C (PI-PLC) family isoforms in astrocytes, individuating in cultured neonatal rat astrocytes, supposed to be quiescent cells, the absence of some isoforms, accordingly to their well known tissue specificity. The same study was conducted in cultured rat astrocytoma C6 cells and designed a different pattern of expression of PI-PLCs in the neoplastic counterpart, accordingly to literature suggesting a PI signalling involvement in tumour progression. It is not clear the role of PI-PLC isoforms in inflammation; recent data demonstrate they are involved in cytokines production, with special regard to IL-6. PI-PLCs expression in LPS treated neonatal rat astrocytes performed by using RT-PCR, observed at 3, 6, 18 and 24 h intervals, expressed: PI-PLC beta1, beta4 and gamma1 in all intervals analysed; PI-PLC delta1 at 6, 18 and 24 h; PI-PLC delta3 at 6 h after treatment. PI-PLC beta3, delta4 and epsilon, present in untreated astrocytes, were not detected after LPS treatment. Immunocytochemical analysis, performed to visualize the sub-cellular distribution of the expressed isoforms, demonstrated different patterns of localisation at different times of exposure. These observations suggest that PI-PLCs expression and distribution may play a role in ongoing inflammation process of CNS.

Phospholipase C as a potential target for cardioprotection during oxidative stressThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Cardiac dysfunction due to ischemia–reperfusion (I/R) is associated with marked changes in membrane function and subsequent Ca2+-handling abnormalities in cardiomyocytes. The membrane abnormalities in hearts subjected to I/R arise primarily from oxidative stress as a consequence of increased formation of reactive oxygen species and other oxidants, as well as reduced antioxidant defenses. Little is known, however, about the nature and mechanisms of the sarcolemmal membrane changes with respect to phospholipase C (PLC)-related signaling events. In addition, the mechanisms involved in protection of the postischemic myocardium and in ischemic preconditioning with respect to PLC function need to be established. Accordingly, this article reviews the historical and current information on PLC-mediated signal transduction mechanisms in I/R, as well as outlining future directions that should be addressed. Such information will extend our knowledge of ischemic heart disease and help improve its therapy.

Suppression of PLCbeta2 by endotoxin plays a role in the adenosine A(2A) receptor-mediated switch of macrophages from an inflammatory to an angiogenic phenotype

Toll-like receptor (TLR) 2, 4, 7, and 9 agonists, together with adenosine A(2A) receptor (A(2A)R) agonists, switch macrophages from an inflammatory (M1) to an angiogenic (M2-like) phenotype. This switch involves induction of A(2A)Rs by TLR agonists, down-regulation of tumor necrosis factor alpha (TNFalpha) and interleukin-12, and up-regulation of vascular endothelial growth factor (VEGF) and interleukin-10 expression. We show here that the TLR4 agonist lipopolysaccharide (LPS) induces rapid and specific post-transcriptional down-regulation of phospholipase C(PLC)beta1 and beta2 expression in macrophages by de-stabilizing their mRNAs. The PLCbeta inhibitor U73122 down-regulates TNFalpha expression by macrophages, and in the presence of A(2A)R agonists, up-regulates VEGF, mimicking the synergistic action of LPS with A(2A)R agonists. Selective down-regulation of PLCbeta2, but not PLCbeta1, using small-interfering RNA resulted in increased VEGF expression in response to A(2A)R agonists, but did not suppress TNFalpha expression. Macrophages from PLCbeta2(-/-) mice also expressed increased VEGF in response to A(2A)R agonists. LPS-mediated suppression of PLCbeta1 and beta2 is MyD88-dependent. In a model of endotoxic shock, LPS (35 microg/mouse, i.p.) suppressed PLCbeta1 and beta2 expression in spleen, liver, and lung of wild-type but not MyD88(-/-) mice. These studies indicate that LPS suppresses PLCbeta1 and beta2 expression in macrophages in vitro and in several tissues in vivo. These results suggest that suppression of PLCbeta2 plays an important role in switching M1 macrophages into an M2-like state.

Involvement of nuclear PLCbeta1 in lamin B1 phosphorylation and G2/M cell cycle progression

Inositide-specific phospholipase Cbeta1 (PLCbeta1) signaling in cell proliferation has been investigated thoroughly in the G(1) cell cycle phase. However, little is known about its involvement in G(2)/M progression. We used murine erythroleukemia cells to investigate the role of PLCbeta1 in G(2)/M cell cycle progression and screened a number of candidate intermediate players, particularly mitogen-activated protein kinase (MAPK) and protein kinase C (PKC), which can, potentially, transduce serum mitogenic stimulus and induce lamin B1 phosphorylation, leading to G(2)/M progression. We report that PLCbeta1 colocalizes and physically interacts with lamin B1. Studies of the effects of inhibitors and selective si-RNA mediated silencing showed a role of JNK, PKCalpha, PKCbetaI, and the beta1 isoform of PI-PLC in cell accumulation in G(2)/M [as observed by fluorescence-activated cell sorter (FACS)]. To shed light on the mechanism, we considered that the final signaling target was lamin B1 phosphorylation. When JNK, PKCalpha, or PLCbeta1 were silenced, lamin B1 exhibited a lower extent of phosphorylation, as compared to control. The salient features to emerge from these studies are a common pathway in which JNK is likely to represent a link between mitogenic stimulus and activation of PLCbeta1, and, foremost, the finding that the PLCbeta1-mediated pathway represents a functional nuclear inositide signaling in the G(2)/M transition.

Combining lipidomics and proteomics of human cerebrospinal fluids

Lipids from dietary sources or from de novo synthesis are transported while bound to proteins to other tissues where they are used for cell membrane synthesis or stored for energy generation. In cell membranes or in plasma, lipids can undergo several modifications that are important in cell function. Several proteins orchestrate the transport, biosynthesis, and modification of lipids. Thus, the intersection of lipids and proteins is important in human metabolic pathways. Recent advances in mass spectrometry and bioinformatics have made it possible to obtain compositional (structural and functional) data of lipid molecular species and proteins in biological samples. This combination of lipidomics and proteomics is advantageous because it allows us to better define biochemical pathways, discover new drug targets, and better understand the pathophysiology of several diseases.

Akt phosphorylation and nuclear phosphoinositide association mediate mRNA export and cell proliferation activities by ALY

Nuclear PI3K and its downstream effectors play essential roles in a variety of cellular activities including cell proliferation, survival, differentiation, and pre-mRNA splicing. Aly is a nuclear speckle protein implicated in mRNA export. Here we show that Aly is a physiological target of nuclear PI3K signaling, which regulates its subnuclear residency, cell proliferation, and mRNA export activities through nuclear Akt phosphorylation and phosphoinositide association. Nuclear Akt phosphorylates Aly on threonine-219, which is required for its interaction with Akt. Aly binds phosphoinositides, and this action is regulated by Akt-mediated phosphorylation. Phosphoinositide binding but not Akt phosphorylation dictates Aly's nuclear speckle residency. Depletion of Aly results in cell growth suppression and mRNA export reduction. Inhibition of Aly phosphorylation substantially decreases cell proliferation and mRNA export. Furthermore, disruption of phosphoinositide association with Aly also significantly reduces these activities. Thus, nuclear PI3K signaling mediates both cell proliferation and mRNA export functions of Aly.

Nuclear sphingolipids: metabolism and signaling

Sphingolipids are most prominently expressed in the plasma membrane, but recent studies have pointed to important signaling and regulatory roles in the nucleus. The most abundant nuclear sphingolipid is sphingomyelin (SM), which occurs in the nuclear envelope (NE) as well as intranuclear sites. The major metabolic product of SM is ceramide, which is generated by nuclear sphingomyelinase and triggers apoptosis and other metabolic changes. Ceramide is further hydrolyzed to free fatty acid and sphingosine, the latter undergoing conversion to sphingosine phosphate by action of a specific nuclear kinase. Gangliosides are another type of sphingolipid found in the nucleus, members of the a-series of gangliotetraose gangliosides (GM1, GD1a) occurring in the NE and endonuclear compartments. GM1 in the inner membrane of the NE is tightly associated with a Na(+)/Ca(2+) exchanger whose activity it potentiates, thereby contributing to regulation of Ca(2+) homeostasis in the nucleus. This was shown to exert a cytoprotective role as absence or inactivation of this nuclear complex rendered cells vulnerable to apoptosis. This was demonstrated in the greatly enhanced kainite-induced seizure activity in knockout mice lacking gangliotetraose gangliosides. The pathology included apoptotic destruction of neurons in the CA3 region of the hippocampus. Ca(2+) homeostasis was restored in these animals with LIGA-20, a membrane-permeant derivative of GM1 that entered the NE and activated the nuclear Na(+)/Ca(2+) exchanger. Some evidence suggests the presence of uncharged glycosphingolipids in the nucleus.

Phospholipid-mediated signaling systems as novel targets for treatment of heart disease

The phospholipases associated with the cardiac sarcolemmal (SL) membrane hydrolyze specific membrane phospholipids to generate important lipid signaling molecules, which are known to influence normal cardiac function. However, impairment of the phospholipases and their related signaling events may be contributory factors in altering cardiac function of the diseased myocardium. The identification of the changes in such signaling systems as well as understanding the contribution of phospholipid-signaling pathways to the pathophysiology of heart disease are rapidly emerging areas of research in this field. In this paper, I provide an overview of the role of phospholipid-mediated signal transduction processes in cardiac hypertrophy and congestive heart failure, diabetic cardiomyopathy, as well as in ischemia-reperfusion. From the cumulative evidence presented, it is suggested that phospholipid-mediated signal transduction processes could serve as novel targets for the treatment of the different types of heart disease.

Expression of phosphoinositide-specific phospholipase C isoenzymes in cultured astrocytes

Signal transduction from plasma membrane to cell nucleus is a complex process depending on various components including lipid signaling molecules, in particular phosphoinositides and their related enzymes, which act at cell periphery and/or plasma membrane as well as at nuclear level. As far as the nervous system may concern the inositol lipid cycle has been hypothesized to be involved in numerous neural as well as glial functions. In this context, however, a precise panel of glial PLC isoforms has not been determined yet. In the present experiments we investigated astrocytic PLC isoforms in astrocytes obtained from foetal primary cultures of rat brain and from an established cultured (C6) rat astrocytoma cell line, two well known cell models for experimental studies on glia. Identification of PLC isoforms was achieved by using a combination of RT-PCR and immunocytochemistry experiments. While in both cell models the most represented PI-PLC isoforms were beta4, gamma1, delta4, and epsilon, isoforms PI-PLC beta2 and delta3 were not detected. Moreover, in primary astrocyte cultures PI-PLC delta3 resulted well expressed in C6 cells but was absent in astrocytes. Immunocytochemistry performed with antibodies against specific PLC isoforms substantially confirmed this pattern of expression both in astrocytes and C6 glioma cells. In particular while some isoenzymes (namely isoforms beta3 and beta4) resulted mainly nuclear, others (isoforms delta4 and epsilon) were preferentially localized at cytoplasmic and plasma membrane level.

Phosphoinositide-specific phospholipase C (PI-PLC) β1 and nuclear lipid-dependent signaling

Over the last years, evidence has suggested that phosphoinositides, which are involved in the regulation of a large variety of cellular processes both in the cytoplasm and in the plasma membrane, are present also within the nucleus. A number of advances has resulted in the discovery that phosphoinositide-specific phospholipase C signalling in the nucleus is involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Even though nuclear inositol lipids hydrolysis generates second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. Among phosphoinositide-specific phospholipase C, the beta(1) isoform appears to be one of the key players of the nuclear lipid signaling. This review aims at highlighting the most significant and up-dated findings about phosphoinositide-specific phospholipase C beta(1) in the nucleus.

Nuclear PI(4,5)P(2): a new place for an old signal

Over the last decades, evidence has accumulated suggesting that there is a distinct nuclear phosphatidylinositol pathway. One of the best examined nuclear lipid pathways is the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) by PLC resulting in activation of nuclear PKC and production of inositol polyphosphates. However, there is a growing number of data that phosphoinositides are not only precursor for soluble inositol phosphates and diacylglycerol, instead they can act as second messengers themselves. They have been implicated to play a role in different important nuclear signaling events such as cell cycle progression, apoptosis, chromatin remodeling, transcriptional regulation and mRNA processing. This review focuses on the role of specifically PI4,5P(2) in the nucleus as a second messenger as well as a precursor for PI3,4,5P3, inositol polyphosphates and diacylglycerol.

Intranuclear 3'-phosphoinositide metabolism and Akt signaling: new mechanisms for tumorigenesis and protection against apoptosis?

Lipid second messengers, particularly those derived from the polyphosphoinositide metabolism, play a pivotal role in multiple cell signaling networks. Phosphoinositide 3-kinase (PI3K) generate 3'-phosphorylated inositol lipids that are key players in a multitude of cell functions. One of the best characterized targets of PI3K lipid products is the serine/threonine protein kinase Akt (protein kinase B, PKB). Recent findings have implicated the PI3K/Akt pathway in tumorigenesis because it stimulates cell proliferation and suppresses apoptosis. However, it was thought that this signal transduction network would exert its carcinogenetic effects mainly by operating in the cytoplasm. Evidence accumulated over the past 15 years has highlighted the presence of an autonomous nuclear inositol lipid cycle, and strongly suggests that lipid molecules are important components of signaling pathways operating at the nuclear level. PI3K, its lipid product phosphatidylinositol (3,4,5) trisphosphate (PtdIns(3,4,5)P3), and Akt have been identified within the nucleus and recent data suggest that they counteract apoptosis also by operating in this cell compartment through a block of caspase-activated DNase and inhibition of chromatin condensation. In this review, we shall summarize the most updated and intriguing findings about nuclear PI3K/PtdIns(3,4,5)P3/Akt in relationship with tumorigenesis and suppression of apoptotic stimuli.

GM1 ganglioside: another nuclear lipid that modulates nuclear calcium. GM1 potentiates the nuclear sodium–calcium exchangerThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

The nuclear envelope (NE) enclosing the cell nucleus, although morphologically and chemically distinct from the plasma membrane, has certain features in common with the latter including the presence of GM1 as an important modulatory molecule. This ganglioside influences Ca2+flux across both membranes, but by quite different mechanisms. GM1 in the NE contributes to regulation of nuclear Ca2+through potentiation of a Na+/Ca2+exchanger in the inner nuclear membrane, whereas in the cell membrane, it regulates cytosolic Ca2+through modulation of a nonvoltage-gated Ca2+channel. Studies with neuroblastoma cells suggest GM1 concentration becomes elevated in the NE with onset of axonogenesis. However, the nuclear GM1/exchanger complex is not limited to neuronal cells but also occurs in NE of astrocytes, C6 cells, and certain non-neural cells. Immunoprecipitation and immunoblot experiments have shown high affinity association of the nuclear Na+/Ca2+exchanger with GM1, in contrast to Na+/Ca2+exchangers of the plasma membrane, which bind GM1 less avidly or not at all. This is believed to be due to different isoforms of the exchanger and a difference in topology of GM1 relative to the large inner loop of the exchanger in the 2 membranes. Cultured neurons from mice genetically engineered to lack GM1 suffered Ca2+dysregulation as seen in their high vulnerability to Ca2+-induced apoptosis. They were rescued by GM1 and more effectively by LIGA20, a membrane-permeant derivative of GM1. The mutant animals were highly susceptible to kainate-induced seizures, which are also a reflection of Ca2+dysregulation. The seizures were effectively attenuated by LIGA20 in parallel with the ability of this agent to enter brain cells, insert into the NE, and potentiate Na+/Ca2+exchange activity in the nucleus. The Na+/Ca2+exchanger of the NE, in association with nuclear GM1, is thus seen contributing to independent regulation of Ca2+by the nucleus in a manner that provides cytoprotection against Ca2+-induced apoptosis.

Nuclear inositol lipid metabolism: more than just second messenger generation?

A distinct polyphosphoinositide cycle is present in the nucleus, and growing evidence suggests its importance in DNA replication, gene transcription, and apoptosis. Even though it was initially thought that nuclear inositol lipids would function as a source for second messengers, recent findings strongly indicate that lipids present in the nucleus also fulfil other roles. The scope of this review is to highlight the most intriguing advances made in the field over the last few years, such as the possibility that nuclear phosphatidylinositol (4,5) bisphosphate is involved in maintaining chromatin in a transcriptionally active conformation, the new emerging roles for intranuclear phosphatidylinositol (3,4,5) trisphosphate and phosphoinositide 3-kinase, and the evidence which suggests a tight relationship between a decreased level of nuclear phosphoinositide specific phospholipase C-beta1 and the evolution of myelodisplastic syndrome into acute myeloid leukemia.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Nuclear phospholipase C beta1, regulation of the cell cycle and progression of acute myeloid leukemia

A large number of observations have hinted at the fact that location impinges on function of some of the main players of nuclear inositol lipid cycle. PLC beta1 is a well-known example, given that it has been shown that only the enzyme located in the nucleus targets the cyclin D3/cdk4 complex, playing, in turn, a key role in the control of normal progression through the G1 phase of the cell cycle. The PLC beta1 gene, which is constituted of 36 small exons and large introns, maps on the short arm of human chromosome 20 (20pl2, nearby markers D20S917 and D20S177) with the specific probe (PAC clone HS881E24) spanning from exon 19 to 32 of the gene itself. The chromosome band 20pl2 has been shown to be rearranged in human diseases such as solid tumors without a more accurate definition of the alteration, maybe because of the absence of candidate genes or specific probes. Moreover, non-specific alterations in chromosome 20 have been found in patients affected by MDS and acute myeloid leukemia AML. MDS is an adult hematological disease that evolves into AML in about 30% of the cases. The availability of a highly specific probe gave an opportunity to perform in patients affected with MDS/AML, associated with normal karyotype, painting and FISH analysis aimed to check the PLC beta1 gene, given that this signaling molecule is a key player in the control of some checkpoints of the normal progression through the cell cycle. FISH analysis disclosed in a small group of MDS/AML patients with normal karyotype the monoallelic deletion of the PLC beta1 gene. In contrast, PLC beta4, another gene coding for a signaling molecule, located on 20pl2.3 at a distance as far as less than 1 Mb from PLC beta1, is unaffected in MDS patients with the deletion of PLC beta1 gene, hinting at an interstitial deletion. The MDS patients, bearing the deletion, rapidly evolved to AML, whilst the normal karyotype MDS patients, showing non-deletion of PLC beta1 gene, are still alive at least 24 months after the diagnosis. The immunocytochemical analysis using an anti PLC beta1 monoclonal antibody showed that all the AML/MDS patients who were normal at FISH analysis also had normal staining of the nucleus, which is a preferential site for PLC beta1. In contrast, the monoallelic deletion gave rise to a dramatic decrease of the nuclear staining suggesting a decreased expression of the nuclear PLC beta1. The reported data strengthen the contention of a key role played by PLC beta1 in the nucleus, suggest a possible involvement of PLC beta1 in the progression of MDS to AML and pave the way for a larger investigation aimed at identifying a possible high risk group among MDS patients with a normal karyotype.

Nucleophosmin/B23, a Nuclear PI(3,4,5)P3 Receptor, Mediates the Antiapoptotic Actions of NGF by Inhibiting CAD

Phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P(3)] is an essential second messenger implicated in various cellular processes. Cytoplasmic PI(3,4,5)P(3) has been well characterized, but little is known about the physiological role of nuclear PI(3,4,5)P(3). Here, we describe a nuclear PI(3,4,5)P(3) receptor, nucleophosmin (NPM)/B23, that mediates the antiapoptotic effects of NGF by inhibiting DNA fragmentation activity of caspase-activated DNase (CAD). Employing PI(3,4,5)P(3) column and NGF-treated PC12 nuclear extracts, we identified B23 as a nuclear PI(3,4,5)P(3) binding protein. Purification from nuclear extract demonstrates that B23 contributes to DNA fragmentation inhibitory activity. Depletion of B23 from nuclear extracts or knockdown B23 in PC12 cells abolishes NGF-provoked protective effect, whereas overexpression of B23 in PC12 cells prevents apoptosis. Further, hydrolyzing PI(3,4,5)P(3) with PTEN or SHIP abrogates its antiapoptotic activity. Moreover, B23 mutants that can not associate with PI(3,4,5)P(3) fail to prevent DNA fragmentation. Thus, the nuclear B23-PI(3,4,5)P(3) complex regulates the antiapoptotic activity of NGF in the nucleus.

DNA chip-based expression profile analysis indicates involvement of the phosphatidylinositol signaling pathway in multiple plant responses to hormone and abiotic treatments

The phosphatidylinositol (PI) metabolic pathway is considered critical in plant responses to many environmental factors, and previous studies have indicated the involvement of multiple PI-related gene families during cellular responses. Through a detailed analysis of the Arabidopsis thaliana genome, 82 polypeptides were identified as being involved in PI signaling. These could be grouped into different families including PI synthases (PIS), PI-phosphate kinases (PIPK), phospholipases (PL), inositol polyphosphate phosphatases (IPPase), inositol polyphosphate kinases (IPK), PI transfer proteins and putative inositol polyphosphate receptors. The presence of more than 10 isoforms of PIPK, PLC, PLD and IPPase suggested that these genes might be differentially expressed during plant cellular responses or growth and development. Accordingly, DNA chip technology was employed to study the expression patterns of various isoforms. In total, 79 mRNA clones were amplified and used for DNA chip generation. Expression profile analysis was performed using samples that represented multiple tissues or cellular responses. Tested samples included normal leaf, stem and flower tissues, and leaves from plants treated with various hormones (auxin, cytokinin, gibberellin, abscisic acid and brassinosteroid) or environmental factors (temperature, calcium, sodium, drought, salicylic acid and jasmonic acid). Results showed that many PI pathway-related genes were differentially expressed under these experimental conditions. In particular, the different isoforms of each family were specifically expressed in many cases, suggesting their involvement in tissue specificity and cellular responses to environmental conditions. This work provides a starting point for functional studies of the relevant PI-related proteins and may help shed light onto the role of PI pathways in development and cellular responses.

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.

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.

PIKE GTPase: a novel mediator of phosphoinositide signaling

Phosphoinositide (PI) 3-kinase enhancer (PIKE) is a brain-specific GTPase that binds to PI 3-kinase and stimulates its lipid kinase activity. It exists in two forms: the first to be identified, PIKE-S, is shorter and exclusively nuclear; by contrast, the longer form, PIKE-L, resides in multiple intracellular compartments. Nerve growth factor treatment leads to PIKE-S activation by triggering the nuclear translocation of phospholipase C (PLC)-γ1, which acts as a physiological guanine nucleotide exchange factor (GEF) for PIKE-S through its Src-homlogy 3 (SH3) domain. Cytoplasmic PI 3-kinase and its lipid product phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] regulate the membrane translocation and activation of many signaling molecules by binding to their pleckstrin homology (PH) domains. However, little is known about the physiological roles of their nuclear counterparts. The nuclear PLC-γ1/PIKE-S/PI 3-kinase signaling pathway seems to be an extension of the crosstalk between cytoplasmic PLC-γ1 and PI 3-kinase. PIKE-L contains a C-terminal extension consisting of an ADP ribosylation-GTPase-activating protein (ArfGAP) domain and two ankyrin repeats in addition to the N-terminal GTPase domain. PIKE-L could have additional, extranuclear functions, including regulation of postsynaptic signaling by metabotropic glutamate receptors.

Nuclear inositides: facts and perspectives

Strong evidence has been accumulating over the last 15 years suggesting that phosphoinositides, which are involved in the regulation of a large variety of cellular processes in the cytoplasm and in the plasma membrane, are present within the nucleus. Several advances have resulted in the discovery that nuclear phosphoinositides are involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Although nuclear inositol lipids generate second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. This review aims at highlighting the most significant and updated findings about inositol lipid metabolism in the nucleus.

Expression of signal transduction proteins during the differentiation of primary human erythroblasts

The high number (>10(8-10)) of primary human pro-erythroblasts (CD36high/CD235alow) obtainable in HEMA culture (Migliaccio et al., 2002) is exploited here to analyse the expression of proteins implicated in erythropoietin (EPO)-signalling (STATs, PI-3K, and PLCs) during the process of erythroid maturation. Human pro-erythroblasts progressed in 4 days of culture with EPO into basophilic- (CD36high/CD235amedium, 24 h), polychromatic-(CD36high/CD235ahigh, 48 h), and, finally, orthochromatic-(CD36low/CD235ahigh, 72-96 h) erythroblasts. During this maturation, STAT-1 was expressed up to the orthochromatic stage, expression of STAT-5, as well as of its target proteins BclxL and IRF1, remained constant up to 48 h (polychromatic-erythroblasts) but decreased by 96 h (orthochromatic-erythroblasts), while that of STAT-3 decreased constantly from 24 h on and became undetectable by 96 h. Expression of PI-3K rapidly decreased with differentiation since only 50% of original protein levels were detected by 48 h. On the other hand, among the members of PLC families investigated, PLC beta4 was not expressed, PLC beta2, delta1, and gamma2 were expressed at constant levels throughout the maturation process, while expression of PLC beta3 and of PLC gamma1 decreased, as PI-3K, by 24 h and that of PLC beta1 was induced by 6 h and became undetectable by 24 h. In conclusion, these data depict the dynamic signalling scenario associated with the maturation of erythroid cells and provide the first indication that members of PLC families (PLC beta1, beta3, and gamma1) might be involved in the control of erythroid differentiation in humans.