Morphological differentiation of N1E-115 neuroblastoma cells by dimethyl sulfoxide activation of lipid second messengers

Microtubule-regulating proteins and cAMP-dependent signaling in neuroblastoma differentiation

Neurons are highly differentiated cells responsible for the conduction and transmission of information in the nervous system. The proper function of a neuron relies on the compartmentalization of their intracellular domains. Differentiated neuroblastoma cells have been extensively used to study and understand the physiology and cell biology of neuronal cells. Here, we show that differentiation of N1E-115 neuroblastoma cells is more pronounced upon exposure of a chemical analog of cyclic AMP (cAMP), db-cAMP. We next analysed the expression of key microtubule-regulating proteins in differentiated cells and the expression and activation of key cAMP players such as EPAC, PKA and AKAP79/150. Most of the microtubule-promoting factors were up regulated during differentiation of N1E-115 cells, while microtubule-destabilizing proteins were down regulated. We observed an increase in tubulin post-translational modifications related to microtubule stability. As expected, db-cAMP increased PKA- and EPAC-dependent signalling. Consistently, pharmacological modulation of EPAC activity instructed cell differentiation, number of neurites, and neurite length in N1E-115 cells. Moreover, disruption of the PKA-AKAP interaction reduced these morphometric parameters. Interestingly, PKA and EPAC act synergistically to induce neuronal differentiation in N1E-115. Altogether these results show that the changes observed in the differentiation of N1E-115 cells proceed by regulating several microtubule-stabilizing factors, and the acquisition of a neuronal phenotype is a process involving concerted although independent functions of EPAC and PKA.

Modulation of the Isoprenoid/Cholesterol Biosynthetic Pathway During Neuronal Differentiation In Vitro

During differentiation, neurons acquire their typical shape and functional properties. At present, it is unclear, whether this important developmental step involves metabolic changes. Here, we studied the contribution of the mevalonate (MVA) pathway to neuronal differentiation using the mouse neuroblastoma cell line N1E-115 as experimental model. Our results show that during differentiation, the activity of 3-hydroxy 3-methylglutaryl Coenzyme A reductase (HMGR), a key enzyme of MVA pathway, and the level of Low Density Lipoprotein receptor (LDLr) decrease, whereas the level of LDLr-related protein-1 (LRP1) and the dimerization of Scavanger Receptor B1 (SRB-1) rise. Pharmacologic inhibition of HMGR by simvastatin accelerated neuronal differentiation by modulating geranylated proteins. Collectively, our data suggest that during neuronal differentiation, the activity of the MVA pathway decreases and we postulate that any interference with this process impacts neuronal morphology and function. Therefore, the MVA pathway appears as an attractive pharmacological target to modulate neurological and metabolic symptoms of developmental neuropathologies. J. Cell. Biochem. 117: 2036-2044, 2016. © 2016 Wiley Periodicals, Inc.

Neuritogenic and neuroprotective effects of polar steroids from the Far East starfishes Patiria pectinifera and Distolasterias nipon

The neuritogenic and neuroprotective activities of six starfish polar steroids, asterosaponin Р₁, (25S)-5α-cholestane-3β,4β,6α,7α,8,15α,16β,26-octaol, and (25S)-5α-cholestane-3β,6α,7α,8,15α,16β,26-heptaol (1–3) from the starfish Patiria pectinifera and distolasterosides D₁–D₃ (4–6) from the starfish Distolasterias nipon were analyzed using the mouse neuroblastoma (NB) C-1300 cell line and an organotypic rat hippocampal slice culture (OHSC). All of these compounds enhanced neurite outgrowth in NB cells. Dose-dependent responses to compounds 1–3 were observed within the concentration range of 10–100 nM, and dose-dependent responses to glycosides 4–6 were observed at concentrations of 1–50 nM. All the tested substances exhibited notable synergistic effects with trace amounts of nerve growth factor (NGF, 1 ng/mL) or brain-derived neurotrophic factor (BDNF, 0.1 ng/mL). Using NB cells and OHSCs, it was shown for the first time that starfish steroids 1–6 act as neuroprotectors against oxygen-glucose deprivation (OGD) by increasing the number of surviving cells. Altogether, these results suggest that neurotrophin-like neuritogenic and neuroprotective activities are most likely common properties of starfish polyhydroxysteroids and the related glycosides, although the magnitude of the effect depended on the particular compound structure.

Optimal neural differentiation and extension of hybrid neuroblastoma cells (NDC) for nerve-target evaluations using a multifactorial approach

In vitro models of tissues, such as the cornea, represent systems for modeling cell-to-cell interactions and tissue function. The objective of this study was to develop an optimized nerve differentiation medium to incorporate into a 3D in vitro model to study innervation and cell targeting. A hybrid neuroblastoma cell line (NDC) was examined for its ability to differentiate into neurons, produce neurites, and functionally contact target cells. Neuronal differentiation of NDCs was optimized through a combinatorial approach which involved culturing cells in the presence of various extracellular matrices and soluble factors. A serum-free medium containing nerve growth factor (NGF), dimethyl sulfoxide (DMSO), or dexamethasone resulted in the greatest proportion of NDCs demonstrating a neuronal morphology. Similarly, with supplementation of cyclic AMP (cAMP) or NGF, neurite extension was optimized. Combining these factors generated an optimized differentiation and extension medium, relative to the individual components alone. In co-culture with epithelial cells, NDC neurites generated in the optimized medium formed contacts with epithelial targets and produced substance P. Similarly, NDCs seeded into a collagen matrix produced neurites that projected through the matrix to target epithelial cells, promoted epithelial stratification, and increased the rate of epithelial wound healing. As well, differentiated NDCs could target and alter acetylcholine receptor clustering in mouse C2C12 myotubes, demonstrating synaptic plasticity. Our data supports the use of NDCs, in combination with optimized medium, for generating an innervated in vitro model.

Differentiation-dependent expression of hypothetical proteins in the neuroblastoma cell line N1E-115

Several protein cascades, including signaling, cytoskeletal, chaperones, metabolic, and antioxidant proteins, have been shown to be involved in the process of neuronal differentiation (ND) of neuroblastoma cell lines. No systematic approach to detect hitherto unknown and unnamed proteins or structures that have been predicted upon nucleic acid sequences in ND has been published so far. We therefore decided to screen hypothetical protein (HP) expression by protein profiling. Two-dimensional gel electrophoresis with subsequent matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF/TOF) identification was used for expression analysis of undifferentiated and dimethylsulfoxide-induced neuronally differentiated N1E-115 cells. We unambiguously identified six HPs: Q8C520, Q99LF4, Q9CXS1, Q9DAF8, Q91WT0, and Q8C5G2. A prefoldin domain in Q91WT0, a t-SNARE domain in Q9CXS1, and a bromodomain were observed in Q8C5G2. For the three remaining proteins, no putative function using Pfam, BLOCKS, PROSITE, PRINTS, InterPro, Superfamily, CoPS, and ExPASy could be assigned. While two proteins were present in both cell lines, Q9CXS1 was switched off (i.e., undetectably low) in differentiated cells only, and Q9DAF8, Q91WT0, and Q8C5G2 were switched on in differentiated cells exclusively. Herein, using a proteomic approach suitable for screening and identification of HP, we present HP structures that have been only predicted so far based upon nucleic acid sequences. The four differentially regulated HPs may play a putative role in the process of ND.

Cytoskeleton changes following differentiation of N1E-115 neuroblastoma cell line

No systematic approach to detect expression of differentiation-related elements was published so far. The undifferentiated N1E-115 neuroblastoma cell line was switched into a neuronal phenotype by DMSO treatment and used for proteomic experiments. We used two-dimensional gel electrophoresis followed by unambiguous mass spectrometrical identification of proteins to generate a map of cytoskeleton proteins (CPs), i.e., to search for differentiation-related structures. Alpha-actin, actin-like protein 6A, gamma-tubulin complex component 2, tubulin alpha 3/alpha 7, CLIP associating protein 2, B4 integrin interactor homolog were detectable in the undifferentiated cell line exclusively and neuron-specific CPs drebrin and presynaptic density protein 95, actin-related protein 2/3, alpha and beta-centractin, PDZ-domain actin binding protein, actinin alpha 1, profilin II, ezrin, coactosin-like protein, transgelin 2, myosin light polypeptide 6, tubulin alpha 2, 6 and 7, beta tubulin (94% similar with tubulin beta-2), tubulin beta 3, tubulin tyrosine ligase-like protein 1, lamin B1 and keratin 20 were observed in the differentiated cell line only. We herein identified differentiation-related expressional patterns thus providing new evidence for the role of CPs in the process of neuronal differentiation.

Capture of extracellular zinc ions by astrocytes

Synaptic zinc ions released during synaptic transmission interact with pre- and postsynaptic neuroreceptors, thus modulating neurotransmission. It is likely that they have to be efficiently cleared from the extracellular milieu to assure subsequent synaptic events. Both neurons and glia are assumed to participate in this clearance by mechanisms that are not fully understood. In this study, electron microscopic zinc cytochemistry has shown zinc-electrondense particles associated with hippocampal astrocytic membranes frequently found accumulated in stacked lamellae. In cultured astrocytes, the use of zinc fluorochromes and endocytic markers allowed the simultaneous imaging of the capture of extracellular zinc simultaneously to plasma membrane markers; this endocytic process was inhibited by high sucrose concentrations. Finally, electron microscopy of zinc-loaded and fluorochrome photoconverted cells demonstrated some early events of extracellular zinc capture as well as its late accumulation in lysosome-like organelles.

The neuronal differentiation process involves a series of antioxidant proteins

Involvement of individual antioxidant proteins (AOXP) and antioxidants in the differentiation process has been already reported. A systematic search strategy for detecting differentially regulated AOXP in neuronal differentiation, however, has not been published so far. The aim of this study was to provide an analytical tool identifying AOXP and to generate a differentiation-related AOXP expressional pattern. The undifferentiated N1E-115 neuroblastoma cell line was switched into a neuronal phenotype by DMSO treatment and used for proteomic experiments: We used two-dimensional gel electrophoresis followed by unambiguous mass spectrometrical (MALDI-TOF-TOF) identification of proteins to generate a map of AOXP. 16 AOXP were unambiguously determined in both cell lines; catalase, thioredoxin domain-containing protein 4 and hypothetical glutaredoxin/glutathione S-transferase C terminus-containing protein were detectable in the undifferentiated cells only. Five AOXP were observed in both, undifferentiated and differentiated cells and thioredoxin, thioredoxin-like protein p19, thioredoxin reductase 1, superoxide dismutases (Mn and Cu-Zn), glutathione synthetase, glutathione S-transferase P1 and Mu1 were detected in differentiated cells exclusively. Herein a differential expressional pattern is presented that reveals so far unpublished antioxidant principles involved in neuronal differentiation by a protein chemical approach, unambiguously identifying AOXP. This finding not only shows concomitant determination of AOXP but also serves as an analytical tool and forms the basis for design of future studies addressing AOXP and differentiation per se.

Glutamate mediates cell death and increases the Bax to Bcl-2 ratio in a differentiated neuronal cell line

Excessive stimulation of the NMDA receptor by glutamate induces cell death and has been implicated in the development of several neurodegenerative diseases. While apoptosis plays a role in glutamate-mediated toxicity, the mechanisms underlying this process have yet to be completely determined. Recent evidence has shown that exposure to excitatory amino acids regulates the expression of the antiapoptotic protein, Bcl-2, and the proapoptotic protein, Bax, in neurons. Since it has been suggested that the ratio of Bax to Bcl-2 is an important determinant of neuronal survival, the reciprocal regulation of these Bcl-2 family proteins may play a role in the neurotoxicity mediated by glutamate. Here, we have used a differentiable neuronal cell line, N1E-115, to investigate the molecular properties of glutamate-induced cell death. Annexin V staining was used to determine apoptotic cell death between 0 and 5 days differentiation with DMSO/low serum. Immunoblot analysis was used to determine whether the expression of Bcl-2 or Bax was modulated during the differentiation process. Bcl-2 protein levels were increased during maturation while Bax expression remained unchanged. Maximum Bcl-2 expression was observed following 5 days of differentiation. Examination of Bcl-2 and Bax following glutamate treatment revealed that the expression of these proteins was inversely regulated. Exposure to glutamate (0.001-10 mM) for 20+/-2 h resulted in a dose-dependent decrease in cell survival (as measured by MTT analysis) that was maximal at 10 mM. These results further support the role of apoptosis in glutamate-mediated cell death. Furthermore, a significant decrease in Bcl-2 levels was observed at 1 mM and 10 mM glutamate (32.1%+/-4.8 and 33.7+/-12.8%, respectively) while a significant upregulation of Bax expression (88.2+/-17.9%) was observed at 10 mM glutamate. Interestingly, Bcl-2 and Bax levels in cells treated with glutamate from 12-24 h were not significantly different from those of control. Taken together, these findings provide additional evidence for the reciprocal regulation of Bcl-2 and Bax expression by glutamate and suggest that neuronal excitotoxicity may, in part, result from the inverse regulation of these proteins.

Activation of phospholipase C increases intramembrane electric fields in N1E-115 neuroblastoma cells

We imaged the intramembrane potential (a combination of transmembrane, surface, and dipole potential) on N1E-115 neuroblastoma cells with a voltage-sensitive dye. After activation of the B(2) bradykinin receptor, the electric field sensed by the dye increased by an amount equivalent to a depolarization of 83 mV. The increase in intramembrane potential was blocked by the phospholipase C (PLC) inhibitors U-73122 and neomycin, and was invariably accompanied by a transient rise of [Ca(2+)](i). A depolarized inner surface potential, as the membrane loses negative charges via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis, and an increase in the dipole potential, as PIP(2) is hydrolyzed to 1,2-diacylglycerol (DAG), can each account for a small portion of the change in intramembrane potential. The primary contribution to the measured change in intramembrane potential may arise from an increased dipole potential, as DAG molecules are generated from hydrolysis of other phospholipids. We found bradykinin produced an inhibition of a M-type voltage-dependent K(+) current (I(K(M))). This inhibition was also blocked by the PLC inhibitors and had similar kinetics as the bradykinin-induced modulation of intramembrane potential. Our results suggest that the change in the local intramembrane potential induced by bradykinin may play a role in mediating the I(K(M)) inhibition.

Secreted phosphatase activity induced by dimethyl sulfoxide in Herpetomonas samuelpessoai

A phosphatase activity of the trypanosomatid parasite Herpetomonas samuelpessoai was characterized using intact living cells. The effects of dimethyl sulfoxide (DMSO) on this activity were investigated. This phosphatase activity (2.53+/-0.01 nmol P(i)/mg protein x min) was linear with cell density and with time for at least 60 min. The optimum pH for the H. samuelpessoai phosphatase lies in the acid range. This phosphatase activity was inhibited by metal chelators and classical phosphatase inhibitors. A robust stimulation of the phosphatase activity was observed when the flagellates were grown in the presence of 4% DMSO, both when intact flagellates and when culture supernatant from those cells were assayed, as observed by biochemical and cytochemical analysis. We also demonstrate that DMSO induced the secretion and/or shedding of this phosphatase to the extracellular medium, with a possible involvement of protein kinase C in this process.

Modeling the pathways of energy balance using the N1E-115 murine neuroblastoma cell line

A good in vitro model within which to investigate molecular interactions between feeding relevant neuropeptide systems has been lacking. Consequently, we began using reverse transcriptase-polymerase chain reaction (RT-PCR) to screen various neuronal cell lines for the presence of feeding relevant neuropeptides and receptors. N1E-115 murine neuroblastoma cells have emerged as an attractive candidate for further analysis because they contain mRNA for a variety of key systems implicated in the regulation of energy homeostasis.

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.

Phospholipase D1 is required for efficient mating projection formation in Saccharomyces cerevisiae

Phospholipase D1 (PLD1) is an important enzyme involved in lipid signal transduction in eukaryotes. A role for PLD1 in signaling in Saccharomyces cerevisiae was examined. Pheromone response in yeast is controlled by a well-characterized protein kinase cascade. Loss of PLD1 activity was found to impair pheromone-induced changes in cellular morphology that result in formation of mating projections. The rate at which projections appeared following pheromone treatment was delayed, suggesting that PLD1 facilitates the execution of a rate-limiting step in morphogenesis. Mutants were found to be less sensitive to pheromone, again arguing that PLD1 is acting at a rate-limiting step. The fact that morphogenesis is most dramatically affected indicates that PLD1 functions primarily in the morphogenic branch of the pheromone response pathway.

Tri-dimensional prostate cell cultures in simulated microgravity and induced changes in lipid second messengers and signal transduction

The high aspect rotating-wall vessel (HARV) was designed to cultivate cells in an environment that simulate microgravity. We studied previously the effects of HARV cultivation on DU-145 human prostate carcinoma cells. We determined that HARV cultivation produced a less aggressive, slower growing, less proliferative, more differentiated and less pliant cell than other cell cultivation methods. The result was a 3-dimensional (3D) growth model of prostate cancer which mimics in vivo tissue growth. This work examines the signal transduction-second messenger pathways existing temporarily in these HARV cells and correlates these features with the special properties in growth and 3D spheroid formation. We found an initial very active ceramide, a diacylglycerol increase together with increases in PI-PLC and PLA(2) a central defect in PLD (no phosphatic acid or phosphatidylethanol at any time during 15 days of HARV cultivation). There is a cross-talk between ceramide and PI3K pathways with activation of PI3K, after 6 days of HARV growth concomitant with down-regulation of ceramide. At this time, there is also an increase of cAMP (seen by increases in arachidonic acid). Taken together these results can explain the 3D organoid-like growth. We therefore developed a model for growth in HARV prostate cancer cells which involve temporal "switches" between second messengers, activation and cross-talk between multiplicity of signaling pathways and a central defect in PLD pathways. Essential to the late slow growth, and 3D organotypic formation are the apoptotic, anti-survival, anti-proliferation and differentiation pathways in the first days of HARV, with growth of "new" different types of prostate cancer cells which set-up for later "switch" in ceramide-PI3K to survival and proliferation.

A direct method for the simultaneous measurement of ceramide and phospholipase D activity

Both ceramide and phospholipase D (PLD) have important roles in a variety of signal transduction pathways. Recent evidence suggests that ceramide is a novel second messenger with specific biological effects. Publications in this field have increased rapidly in the last few years. However, a method to directly and rapidly measure cermide production has been lacking. Herein, we report on a novel, inexpensive, direct and rapid assay for the measurement of ceramide and the simultaneous measurement of PLD activity. This method uses labeling of cells with [(14)C]myristic acid and a TLC solvent of ethyl acetate/acetic acid/trimethylpentane. This method avoids the loss of radioactivity and variability due to changes in DAG kinase activity that are associated with the commonly-used DAG kinase assay.

Cross-talk between phosphatidylinositol 3-kinase and sphingomyelinase pathways as a mechanism for cell survival/death decisions

Peptide hormones act to regulate apoptosis through activation of multiple pro- and anti-apoptotic signaling cascades of which lipid signaling events represent an important facet of the cellular rheostat that determines survival and death decisions. Activation of sphingomyelinase, which generates ceramide, is an intermediate in cellular stress responses and induction of apoptosis in many systems. Conversely, phosphatidylinositol 3-kinase (PI3K) is a critical signaling molecule involved in regulating cell survival and proliferation pathways. In the present study, we investigate cross-talk between the PI3K and sphingomyelinase pathways as a mechanism for regulation of cell survival/death decisions. We show that phorbol ester, insulin-like growth factor 1, and a constitutively active PI3K suppress both tumor necrosis factor-induced apoptosis and ceramide generation. Conversely, inhibition of the PI3K pathway with expression of a kinase-dead PI3K both prevented survival signaling and enhanced tumor necrosis factor-induced ceramide generation. The ability of exogenous sphingomyelinase to induce ceramide generation was partially suppressed by expression of constitutively active PI3K and enhanced by inhibition of PI3K suggesting that cross-talk between PI3K and ceramide generation within cells is regulated subsequent to activation of sphingomyelinase.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis in rat-2 fibroblasts by cell-permeable ceramides

Phospholipids and sphingolipids are important precursors of lipid-derived second messengers such as diacylglycerol and ceramide, which participate in several signal transduction pathways and in that way mediate the effects of various agonists. The cross-talk between glycerophospholipid and sphingolipid metabolism was investigated by examining the effects of cell-permeable ceramides on phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) synthesis in Rat-2 fibroblasts. Addition of short-chain C6-ceramide to the cells resulted in a dose- and time-dependent inhibition of the CDP-pathways for PtdCho and PtdEtn synthesis. Treatment of cells for 4 h with 50 microM C6-ceramide caused an 83% and a 56% decrease in incorporation of radiolabelled choline and ethanolamine into PtdCho and PtdEtn, respectively. Exposure of the cells for longer time-periods (>/= 16 h) to 50 microM C6-ceramide resulted in apoptosis. The structural analogue dihydro-C6-ceramide did not affect PtdCho and PtdEtn synthesis. In pulse-chase experiments, radioactive choline and ethanolamine accumulated in CDP-choline and CDP-ethanolamine under the influence of C6-ceramide, suggesting that synthesis of both PtdCho and PtdEtn were inhibited at the final step in the CDP-pathways. Indeed, cholinephosphotransferase and ethanolaminephosphotransferase activities in membrane fractions from C6-ceramide-treated cells were reduced by 64% and 43%, respectively, when compared with control cells. No changes in diacylglycerol mass levels or synthesis of diacylglycerol from radiolabelled palmitate were observed. It was concluded that C6-ceramide affected glycerophospholipid synthesis predominantly by inhibition of the step in the CDP-pathways catalysed by cholinephosphotransferase and ethanolaminephosphotransferase.

Modification of membrane currents in mouse neuroblastoma cells following infection with rabies virus

1. The effect on membrane currents of infection of mouse neuroblastoma NA cells with rabies virus was studied by using the whole-cell patch clamp technique. 2. Three types of membrane currents, namely voltage-dependent Na+ current (I(Na)), delayed rectifier K+ current (I(K-DR)) and inward rectifier K+ current (I(K-IR)) were elicited in uninfected cells. 3. In cells 3 days after infection with the virus, no detectable change was observed in morphology and membrane capacitance, but I(Na) and I(K-IR) were significantly decreased in amplitude without any appreciable difference in the time course of current activation and inactivation. The voltage-dependence of I(Na) activation was significantly shifted in the positive direction along the voltage axis with a decreased slope. I(K-DR) remained almost unaltered after the viral infection. 4. The resting membrane potential, measured with a physiological K+ gradient across the cell membrane, was decreased (depolarized) after the viral infection. The depolarization was associated with the decreased amplitude of I(K-IR). 5. These results suggest that infection of mouse neuroblastoma NA cells with rabies virus causes reduction of functional expression of ion channels responsible for I(Na) and I(K-IR), and provide evidence for possible involvement of the change in membrane properties in the pathogenesis of rabies disease.

Bradykinin inhibits ceramide production and activates phospholipase D in rabbit cortical collecting duct cells

Recent reports suggest that inflammatory cytokines, growth factors, and vasoconstrictor peptides induce sphingomyelinase (SMase) activity. This results in the hydrolysis of sphingomyelin (SM) into ceramide, which is implicated in various cellular functions. Although ceramide regulates phospholipase D (PLD) activity, there is controversy about this relationship. Thus we investigated whether the effect of bradykinin (BK), a proinflammatory factor and vasodilator, was mediated by ceramide signal transduction and by PLD. In rabbit cortical collecting duct (RCCD) cells, BK increased SM levels and decreased ceramide levels in a time-dependent manner. Thus SMase activity was inhibited by BK. Also, the production of ceramide was regulated in a concentration-dependent manner. The BK-B1 antagonist [Lys-des-Arg9,Leu8]BK did not affect ceramide signal transduction but the BK-B2 antagonist (Hoe-140) blocked the effect of BK on SMase, suggesting that the BK-B2 receptor mediates BK-induced inhibition of ceramide generation. Our results show that exogenous SMase significantly hydrolyzed endogenous SM to form ceramide and weakly activated PLD. In contrast, BK induced a significant activation of PLD. However, additive effects of BK and ceramide on PLD activity were not observed. We concluded that in RCCD cells, the BK-induced second messengers ceramide and phosphatidic acid were generated by distinct signal transduction mechanisms, namely the SMase and PLD pathways.

Granulocyte-Macrophage Colony-Stimulating Factor Rescues TF-1 Leukemia Cells From Ionizing Radiation-Induced Apoptosis Through a Pathway Mediated by Protein Kinase Cα

Protein kinase C (PKC) activity has a recognized role in mediating apoptosis. However, the role of individual PKC isoforms in apoptosis is poorly defined. Therefore, we investigated the translocation of individual PKC isoforms during radiation-induced apoptosis with and without rescue from apoptosis by granulocyte-macrophage colony-stimulating factor (GM-CSF) in the human erythroleukemia cell line TF-1. PKCα was translocated from the particulate to cytosolic fraction of TF-1 cells within 5 minutes of treatment with apoptosis-inducing levels of ionizing radiation. However, this postirradiation translocation did not occur when cells were rescued from apoptosis by GM-CSF. Furthermore, treatment of cells with Gö6976, an inhibitor of classical PKC isoforms, abrogated the rescue effect of GM-CSF. The calcium-independent novel PKC isoform, PKCδ appeared to be degraded in both the particulate and cytosolic fractions of TF-1 cells after treatment with apoptosis-inducing levels of ionizing radiation in either the presence or absence of GM-CSF rescue. Levels of ceramide, a lipid mediator of apoptosis, were measured at 2, 4, 8, 10, and 60 minutes after treatment with ionizing radiation and were substantially reduced in TF-1 cells rescued from apoptosis by GM-CSF compared with apoptotic TF-1 cells. The largest decrease in ceramide production seen was at 4 minutes postirradiation, with a 46% reduction in ceramide levels in TF-1 cells rescued from apoptosis by GM-CSF compared with those in apoptotic TF-1 cells. Because ceramide has been shown to affect PKCα subcellular distribution, these data implicate a role for ceramide in mediating the rapid postirradiation translocation and inhibition of PKCα in TF-1 cells not rescued from apoptosis by GM-CSF. Expression of the antiapoptotic protein Bcl-2 doubled in TF-1 cells rescued from apoptosis by GM-CSF, but did not increase in unrescued cells. Our findings suggest that activated PKCα and increased expression of Bcl-2 after γ irradiation determine survival in TF-1 cells rescued from apoptosis with GM-CSF and that PKCδ plays a role in mediating signals involved in sensing cellular damage and/or regulation of cell damage repair.

Granulocyte-Macrophage Colony-Stimulating Factor Rescues TF-1 Leukemia Cells From Ionizing Radiation-Induced Apoptosis Through a Pathway Mediated by Protein Kinase Cα

Protein kinase C (PKC) activity has a recognized role in mediating apoptosis. However, the role of individual PKC isoforms in apoptosis is poorly defined. Therefore, we investigated the translocation of individual PKC isoforms during radiation-induced apoptosis with and without rescue from apoptosis by granulocyte-macrophage colony-stimulating factor (GM-CSF) in the human erythroleukemia cell line TF-1. PKCα was translocated from the particulate to cytosolic fraction of TF-1 cells within 5 minutes of treatment with apoptosis-inducing levels of ionizing radiation. However, this postirradiation translocation did not occur when cells were rescued from apoptosis by GM-CSF. Furthermore, treatment of cells with Gö6976, an inhibitor of classical PKC isoforms, abrogated the rescue effect of GM-CSF. The calcium-independent novel PKC isoform, PKCδ appeared to be degraded in both the particulate and cytosolic fractions of TF-1 cells after treatment with apoptosis-inducing levels of ionizing radiation in either the presence or absence of GM-CSF rescue. Levels of ceramide, a lipid mediator of apoptosis, were measured at 2, 4, 8, 10, and 60 minutes after treatment with ionizing radiation and were substantially reduced in TF-1 cells rescued from apoptosis by GM-CSF compared with apoptotic TF-1 cells. The largest decrease in ceramide production seen was at 4 minutes postirradiation, with a 46% reduction in ceramide levels in TF-1 cells rescued from apoptosis by GM-CSF compared with those in apoptotic TF-1 cells. Because ceramide has been shown to affect PKCα subcellular distribution, these data implicate a role for ceramide in mediating the rapid postirradiation translocation and inhibition of PKCα in TF-1 cells not rescued from apoptosis by GM-CSF. Expression of the antiapoptotic protein Bcl-2 doubled in TF-1 cells rescued from apoptosis by GM-CSF, but did not increase in unrescued cells. Our findings suggest that activated PKCα and increased expression of Bcl-2 after γ irradiation determine survival in TF-1 cells rescued from apoptosis with GM-CSF and that PKCδ plays a role in mediating signals involved in sensing cellular damage and/or regulation of cell damage repair.

Differential effects of dimethyl sulfoxide on nicotinic acetylcholine receptors from mouse muscle and Torpedo electrocytes

The present study examined the effects of 0.1% dimethyl sulfoxide (DMSO) on nicotinic acetylcholine receptors (nAChR) from mouse muscle and Torpedo californica electrocytes. Receptors were expressed in Xenopus laevis oocytes and studied with voltage-clamp. When applied simultaneously with acetylcholine, DMSO did not inhibit current amplitude of either receptor. Preincubation with DMSO for 1 min reduced current amplitude by approximately 50% from oocytes expressing electrocyte receptor. Preincubation did not affect the muscle receptor. With electric organ membranes, 0.1% DMSO did not block either [alpha-(125)I]bungarotoxin binding to the nAChR agonist site or [3H]phencyclidine binding to its high affinity site on resting or desensitized receptor. These data suggest that DMSO might be affecting the electrocyte receptor through a second messenger system.

Coordinated effects of electromagnetic field exposure on erythropoietin-induced activities of phosphatidylinositol-phospholipase C and phosphatidylinositol 3-kinase

Initial studies with the erythropoietin-sensitive human hematopoietic cell line, TF1, demonstrated both multifarious effects of pulsed electromagnetic field (EMF) exposure on lipid signal transduction and antiproliferative effects of EMF. Stimulation of TF1 cells with erythropoietin resulted in increased phosphatidylinositol 3-kinase activity within 2 min. Addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, produced a decrease in cell proliferation as measured by accumulation of cells in the G0/G1 phase of the cell cycle and suppression of erythropoietin-induced DNA synthesis. Similar effects on cell proliferation were seen under EMF treatment. Phosphatidylinositol 3-kinase activity in erythropoietin-stimulated TF1 cells, measured in whole-cell extracts, increased 34% within 2 min and remained above basal levels for at least 20 min. EMF decreased erythropoietin-stimulated phosphatidylinositol 3-kinase activity to lower than basal levels. Additionally, translocation of the 85-kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase to the membrane was prevented by EMF. Phosphatidylinositol-specific phospholipase C was activated, as reflected by increases in diacylglycerol and inositol trisphosphate at 15-60 s after EMF treatment. These results provide the first evidence of subtle coordinated changes by EMF associated with loss of phosphatidylinositol 3-kinase activity, inhibition of the translocation of p85 to the membrane, and activation of phosphatidylinositol-phospholipase C.