Sphingolipids of the nucleus and their role in nuclear signaling

Transcriptional Differences in Lipid-Metabolizing Enzymes in Murine Sebocytes Derived from Sebaceous Glands of the Skin and Preputial Glands

Sebaceous glands are adnexal structures, which critically contribute to skin homeostasis and the establishment of a functional epidermal barrier. Sebocytes, the main cell population found within the sebaceous glands, are highly specialized lipid-producing cells. Sebaceous gland-resembling tissue structures are also found in male rodents in the form of preputial glands. Similar to sebaceous glands, they are composed of lipid-specialized sebocytes. Due to a lack of adequate organ culture models for skin sebaceous glands and the fact that preputial glands are much larger and easier to handle, previous studies used preputial glands as a model for skin sebaceous glands. Here, we compared both types of sebocytes, using a single-cell RNA sequencing approach, to unravel potential similarities and differences between the two sebocyte populations. In spite of common gene expression patterns due to general lipid-producing properties, we found significant differences in the expression levels of genes encoding enzymes involved in the biogenesis of specialized lipid classes. Specifically, genes critically involved in the mevalonate pathway, including squalene synthase, as well as the sphingolipid salvage pathway, such as ceramide synthase, (acid) sphingomyelinase or acid and alkaline ceramidases, were significantly less expressed by preputial gland sebocytes. Together, our data revealed tissue-specific sebocyte populations, indicating major developmental, functional as well as biosynthetic differences between both glands. The use of preputial glands as a surrogate model to study skin sebaceous glands is therefore limited, and major differences between both glands need to be carefully considered before planning an experiment.

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.

Effects of a Sudden Drop in Salinity on Scapharca subcrenata Antioxidant Defenses and Metabolism Determined Using LC-MS Non-targeted Metabolomics

In this experiment, the effects of a sudden drop in salinity on the antioxidant defense system and related gene expression of the ark shell Scapharca subcrenata were examined. The sudden drop in seawater salinity after a rainstorm was simulated, and subsequently differentially expressed metabolic markers were identified by LC-MS non-targeted metabolomics. When the salinity dropped to 14‰ (S14), the total anti-oxidant content, activity of Na⁺/K⁺-ATPase, superoxide dismutase (SOD), and catalase (CAT), content of malondialdehyde, and expression levels of Mn-SOD, CAT, and C-type lectin of S. subcrenata were significantly higher than in groups with salinity of 22‰ (S22) or 30‰ (S30) (P < 0.05). The activity of glutathione peroxidase (GPx), the content of reduced glutathione, and the expression levels of GP_x were not significantly different between S14 and S22, but the values in each group were significantly higher than those in S30 (P < 0.05). Using the metabolomics technique, 361, 271, and 264 metabolites with significant differences were identified from S22 vs. S14, S30 vs. S14, and S30 vs. S22, respectively. The drop in salinity was accompanied by up-regulation of phosphatidylcholine (PC) (20:4 (5Z, 8Z, 11Z, 14Z)/P-18: 1 (11Z)), PC (16:0/22: 6 (4Z, 7Z, 10Z, 13Z, 16Z, 19Z)), phosphatidylethanolamine (PE) (18:4 (6Z, 9Z, 12Z, 15Z)/24:1 (15Z)), phosphatidylinositol (PI) (20:1 (11Z)/0:0), phalluside-1, C16 sphinganine, and LacCer (d18:0/14:0) and by significant down-regulation of PI-Cer (d18:1/14:0) and PE (14:0/16:1(9Z). The results of this study illustrate how these nine metabolites can be used as metabolic markers for the response of S. subcrenata to a sudden drop in salinity. They also provide the theoretical groundwork for selection of bottom areas with salinity that is optimal for release and proliferation of S. subcrenata, which is needed to restore the declining populations of this species.

Glucosylceramide acyl chain length is sensed by the glycolipid transfer protein

The glycolipid transfer protein, GLTP, can be found in the cytoplasm, and it has a FFAT-like motif (two phenylalanines in an acidic tract) that targets it to the endoplasmic reticulum (ER). We have previously shown that GLTP can bind to a transmembrane ER protein, vesicle-associated membrane protein-associated protein A (VAP-A), which is involved in a wide range of ER functions. We have addressed the mechanisms that might regulate the association between GLTP and the VAP proteins by studying the capacity of GLTP to recognize different N-linked acyl chain species of glucosylceramide. We used surface plasmon resonance and a lipid transfer competition assay to show that GLTP prefers shorter N-linked fully saturated acyl chain glucosylceramides, such as C8, C12, and C16, whereas long C18, C20, and C24-glucosylceramides are all bound more weakly and transported more slowly than their shorter counterparts. Changes in the intrinsic GLTP tryptophan fluorescence blueshifts, also indicate a break-point between C16- and C18-glucosylceramide in the GLTP sensing ability. It has long been postulated that GLTP would be a sensor in the sphingolipid synthesis machinery, but how this mechanistically occurs has not been addressed before. It is unclear what proteins the GLTP VAP association would influence. Here we found that if GLTP has a bound GlcCer the association with VAP-A is weaker. We have also used a formula for identifying putative FFAT-domains, and we identified several potential VAP-interactors within the ceramide and sphingolipid synthesis pathways that could be candidates for regulation by GLTP.

Nuclear lipid mediators: Role of nuclear sphingolipids and sphingosine-1-phosphate signaling in epigenetic regulation of inflammation and gene expression

Phospholipids, sphingolipids, and cholesterol are integral components of eukaryotic cell organelles, including the nucleus. Recent evidence shows characteristic features of nuclear lipid composition and signaling, which are distinct from that of the cytoplasm and plasma membrane. While the nuclear phosphoinositol lipid signaling in cell cycle regulation and differentiation has been well described, there is a paucity on the role of nuclear sphingolipids and sphingolipid signaling in different physiological and pathophysiological human conditions. In this prospective, we describe the role of sphingolipids and specifically focus on the sphingoid bases, such as sphingosine, ceramide, and sphingosine-1-phosphate (S1P) generation and catabolism in nuclear signaling and function. Particularly, S1P generated in the nucleus by phosphorylation of SPHK2 modulates HDAC activity either by direct binding or through activation of nuclear reactive oxygen species and regulates cell cycle and pro-inflammatory gene expression. Potential implication of association of SPHK2 with the co-repressor complexes and generation of S1P in the nucleus on chromatin remodeling under normal and pathological conditions is discussed. A better understanding of sphingolipid signaling in the nucleus will facilitate the design and development of new and novel therapeutic approaches to modulate expression of pro-inflammatory and cell cycle dependent genes in human pathologies such as cancer, bacterial lung infection, neurodegeneration, and cystic fibrosis.

Glycolipids and Lectins in Endocytic Uptake Processes

A host of endocytic processes has been described at the plasma membrane of eukaryotic cells. Their categorization has most commonly referenced cytosolic machinery, of which the clathrin coat has occupied a preponderant position. In what concerns intra-membrane constituents, the focus of interest has been on phosphatidylinositol lipids and their capacity to orchestrate endocytic events on the cytosolic leaflet of the membrane. The contribution of extracellular determinants to the construction of endocytic pits has received much less attention, depite the fact that (glyco)sphingolipids are exoplasmic leaflet fabric of membrane domains, termed rafts, whose contributions to predominantly clathrin-independent internalization processes is well recognized. Furthermore, sugar modifications on extracellular domains of proteins, and sugar-binding proteins, termed lectins, have also been linked to the uptake of endocytic cargoes at the plasma membrane. In this review, we first summarize these contributions by extracellular determinants to the endocytic process. We thus propose a molecular hypothesis - termed the GL-Lect hypothesis - on how GlycoLipids and Lectins drive the formation of compositional nanoenvrionments from which the endocytic uptake of glycosylated cargo proteins is operated via clathrin-independent carriers. Finally, we position this hypothesis within the global context of endocytic pathway proposals that have emerged in recent years.

X-ray reflectivity and grazing incidence diffraction studies of interaction between human adhesion/growth-regulatory galectin-1 and DPPE-GM1 lipid monolayer at an air/water interface

The specific interaction of ganglioside GM1 with the homodimeric (prototype) endogenous lectin galectin-1 triggers growth regulation in tumor and activated effector T cells. This proven biorelevance directed interest to studying association of the lectin to a model surface, i.e. a 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine/ganglioside GM1 (80 : 20 mol%) monolayer, at a bioeffective concentration. Surface expansion by the lectin insertion was detected at a surface pressure of 20 mN/m. On combining the methods of grazing incidence X-ray diffraction and X-ray reflectivity, a transient decrease in lipid-ordered phase of the monolayer was observed. The measured electron density distribution indicated that galectin-1 is oriented with its long axis in the surface plane, ideal for cis-crosslinking. The data reveal a conspicuous difference to the way the pentameric lectin part of the cholera toxin, another GM1-specific lectin, is bound to the monolayer. They also encourage further efforts to monitor effects of structurally different members of the galectin family such as the functionally antagonistic chimera-type galectin-3.

Distinguishing nuclei-specific benzo[a]pyrene-induced effects from whole-cell alterations in MCF-7 cells using Fourier-transform infrared spectroscopy

Exposure to chemicals such as benzo[a]pyrene (B[a]P) can generate intracellular toxic mechanisms. Fourier-transform infrared (FTIR) spectroscopy is a novel approach that allows the non-destructive analysis of underlying chemical bond alterations in patho-physiological processes. This study set out to examine whether B[a]P-induced whole cell alterations could be distinguished from effects on nuclei of exposed cells. Using attenuated total reflection FTIR (ATR-FTIR) spectroscopy, alterations in nuclei isolated from B[a]P-treated MCF-7 cells concentrated either in G0/G1- or S-phase were observed. B[a]P-induced effects in whole-cells included alterations to lipids, DNA and protein spectral regions. Absorbance areas for protein and DNA/RNA regions in B[a]P-treated whole cells differed significantly (P<0.0001) from vehicle controls and these observations correlated with alterations noted in isolated nuclei. Our findings provide evidence that FTIR spectroscopy has the ability to identify specific chemical-induced alterations.

Activation of muscarinic receptors in rat parotid acinar cells induces AQP5 trafficking to nuclei and apical plasma membrane

BACKGROUND

The subcellular distribution of aquaporin-5 (AQP5) in rat parotid acinar cells in response to muscarinic acetylcholine receptor (mAChR) activation remains unclear.

METHODS

Immunoconfocal and immunoelectron microscopy were used to visualize the distribution of AQP5 in parotid acinar cells. Western blotting was used to analyze AQP5 levels in membranes. To clarify the characteristics of membrane domains associated with AQP5, detergent solubility and sucrose-density flotation experiments were performed.

RESULTS

Under control conditions, AQP5 was diffusely distributed on the apical plasma membrane (APM) and apical plasmalemmal region and throughout the cytoplasm. Upon mAChR activation, AQP5 was predominantly located in the nucleus, APM and lateral plasma membrane (LPM). Subsequently, localization of AQP5 in the nucleus, APM and LPM was decreased. Prolonged atropine treatment inhibited mAChR agonist-induced translocation of AQP5 to the nucleus, APM and LPM. AQP5 levels were enhanced in isolated nuclei and nuclear membranes prepared from parotid tissues incubated with mAChR agonist. mAChR agonist induced AQP5 levels in both soluble and insoluble nuclear fractions solubilized with Triton X-100 or Lubrol WX. Small amounts of AQP5 in nuclei were detected using low-density sucrose gradient. When AQP5 was present in the nuclear membrane, nuclear size decreased.

CONCLUSION

The activation of mAChR induced AQP5 translocation to the nucleus, APM and LPM, and AQP5 may trigger water transport across the nuclear membrane and plasma membrane in rat parotid acinar cells.

GENERAL SIGNIFICANCE

AQP5 translocates to the nuclear membrane and may trigger the movement of water, inducing shrinkage of the nucleus and the start of nuclear functions.

Lipidomics in the study of lipid metabolism: Current perspectives in the omic sciences

The advances in systems biology and in the development of new technological tools in analysis, as well as in the omic sciences, among which, metabolomics, and more specifically, lipidomics, have made it possible to investigate the structural and functional complexity of lipids in biological systems. Liquid chromatography and mass spectrometry are the analytical approaches most used in lipid research. Biomedical research, with the development of specific markers for lipids, together with new software development, have both enabled the early diagnosis of several illnesses, besides the evaluation of drug activity and treatment efficacy.

Nuclear lipid microdomain as place of interaction between sphingomyelin and DNA during liver regeneration

Nuclear sphingomyelin is a key molecule for cell proliferation. This molecule is organized with cholesterol and proteins to form specific lipid microdomains bound to the inner nuclear membrane where RNA is synthesized. Here, we have reported the ability of the sphingomyelin present in the nuclear microdomain to bind DNA and regulate its synthesis, and to highlight its role in cell proliferation induced by partial hepatectomy. During G1/S transition of the cell cycle, sphingomyelin and DNA content is very high and it is strongly reduced after exogenous sphingomyelinase treatment. During the S-phase of the cell cycle, the stimulation of sphingomyelinase and inhibition of sphingomyelin-synthase are accompanied by the DNA synthesis start. To assess the specificity of the results, experiments were repeated with trifluoperazine, a drug known to affect the synthesis of lipids and DNA and to stimulate sphingomyelinase activity. The activity of sphingomyelinase is stimulated in the first hour after hepatectomy and sphingomyelin-DNA synthesis is strongly attenuated. It may be hypothesized that the nuclear microdomain represents a specific area of the inner nuclear membrane that acts as an active site of chromatin anchorage thanks to the stabilizing action of sphingomyelin. Thus, sphingomyelin metabolism in nuclear lipid microdomains is suggested to regulate cell proliferation.

Essential roles of gangliosides in the formation and maintenance of membrane microdomains in brain tissues

Gangliosides are considered to be involved in the maintenance and repair of nervous tissues. Recently, novel roles of gangliosides in the regulation of complement system were reported. Here we summarized roles of gangliosides in the formation and maintenance of membrane microdomains in brain tissues by comparing complement activation, inflammatory reaction and disruption of glycolipid-enriched microdomain (GEM)/rafts among several mutant mice of ganglioside synthases. Depending on the defects in ganglioside compositions, corresponding up-regulation of complement-related genes, proliferation of astrocytes and infiltration of microglia were found with gradual severity. Immunoblotting of fractions separated by sucrose density gradient ultracentrifugation revealed that DAF and NCAM having GPI-anchors tended to disappear from the raft fraction with intensities of DKO > GM2/GD2 synthase KO > GD3 synthase KO > WT. The lipid raft markers tended to disperse from the raft fractions with similar intensities. Phospholipids and cholesterol also tended to decrease in GEM/rafts in GM2/GD2 synthase KO and DKO, although total amounts were almost equivalent. All these results indicate that GEM/rafts architecture is destroyed by ganglioside deficiency with gradual intensity depending on the degree of defects of their compositions. Implication of inflammation caused by deficiency of gangliosides in various neurodegenerative diseases was discussed.

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.

Sphingolipids and cancer: ceramide and sphingosine-1-phosphate in the regulation of cell death and drug resistance

Sphingolipids have emerged as bioeffector molecules, controlling various aspects of cell growth and proliferation in cancer, which is becoming the deadliest disease in the world. These lipid molecules have also been implicated in the mechanism of action of cancer chemotherapeutics. Ceramide, the central molecule of sphingolipid metabolism, generally mediates antiproliferative responses, such as cell growth inhibition, apoptosis induction, senescence modulation, endoplasmic reticulum stress responses and/or autophagy. Interestingly, recent studies suggest de novo-generated ceramides may have distinct and opposing roles in the promotion/suppression of tumors, and that these activities are based on their fatty acid chain lengths, subcellular localization and/or direct downstream targets. For example, in head and neck cancer cells, ceramide synthase 6/C(16)-ceramide addiction was revealed, and this was associated with increased tumor growth, whereas downregulation of its synthesis resulted in ER stress-induced apoptosis. By contrast, ceramide synthase 1-generated C(18)-ceramide has been shown to suppress tumor growth in various cancer models, both in situ and in vivo. In addition, ceramide metabolism to generate sphingosine-1-phosphate (S1P) by sphingosine kinases 1 and 2 mediates, with or without the involvement of G-protein-coupled S1P receptor signaling, prosurvival, angiogenesis, metastasis and/or resistance to drug-induced apoptosis. Importantly, recent findings regarding the mechanisms by which sphingolipid metabolism and signaling regulate tumor growth and progression, such as identifying direct intracellular protein targets of sphingolipids, have been key for the development of new chemotherapeutic strategies. Thus, in this article, we will present conclusions of recent studies that describe opposing roles of de novo-generated ceramides by ceramide synthases and/or S1P in the regulation of cancer pathogenesis, as well as the development of sphingolipid-based cancer therapeutics and drug resistance.

New findings on nuclear gangliosides: overview on metabolism and function

GM1 and GD1a gangliosides occur in both membranes of the nuclear envelope (NE) together with two isoforms of neuraminidase. The Neu3 isoform of neuraminidase occurs in the inner membrane of the NE and Neu1 in the outer membrane. Both isoforms convert GD1a to GM1 within the respective membranes. GM1 in the inner membrane is tightly associated with a Na(+) /Ca(2+) exchanger (NCX) and potentiates the latter's activity. The NCX/GM1 complex mediates transfer of nucleoplasmic Ca(2+) to the NE lumen and hence to the endoplasmic reticulum (ER) with which it is continuous. Since cytoplasmic- and nucleoplasmic Ca(2+) are in homeostatic equilibrium (via nuclear pores), the nuclear NCX/GM1 complex acts to gate Ca(2+) transfer from cytosol to ER via nucleoplasm and NE. This constitutes an alternate route to the SERCA pump, indicating the influence of nuclear NCX/GM1 on whole cell Ca(2+) homeostasis. Use of cameleon-fluorescent Ca(2+) indicators (R. Tsien) demonstrated no Ca(2+) transfer from cytosol/nucleoplasm to ER in cells lacking nuclear NCX (Jurkat), and significantly reduced Ca(2+) transfer in cells lacking nuclear GM1 (NG-CR72). NCX/GM1 appears in the NE of neurons as they differentiate and serves a cytoprotective function, as seen in the high susceptibility of GalNAcT-/- knockout mice to kainate-induced seizure activity. This was alleviated by intraperitoneal injections of LIGA-20 a derivative of GM1 that is able (unlike GM1 itself) to traverse the blood brain barrier and neuronal plasma membrane and insert into the NE where it restores NCX exchanger activity. Absence or loss of nuclear GM1 renders cells vulnerable to apoptotic elimination.

The interplay between bioactive sphingolipids and steroid hormones

Steroid hormones regulate various physiological processes including development, reproduction, and metabolism. These regulatory molecules are synthesized from cholesterol in endocrine organs - such as the adrenal glands and gonads - via a multi-step enzymatic process that is catalyzed by the cytochrome P450 superfamily of monooxygenases and hydroxysteroid dehydrogenases. Steroidogenesis is induced by trophic peptide hormones primarily via the activation of a cAMP/protein kinase A (PKA)-dependent pathway. However, other signaling molecules, including cytokines and growth factors, control the steroid hormone biosynthetic pathway. More recently, sphingolipids, including ceramide, sphingosine-1-phosphate, and sphingosine, have been found to modulate steroid hormone secretion at multiple levels. In this review, we provide a brief overview of the mechanisms by which sphingolipids regulate steroidogenesis. In addition, we discuss how steroid hormones control sphingolipid metabolism. Finally, we outline evidence supporting the emerging role of bioactive sphingolipids in various nuclear processes and discuss a role for nuclear sphingolipid metabolism in the control of gene transcription.

Lipid Mediators in the Neural Cell Nucleus: Their Metabolism, Signaling, and Association with Neurological Disorders

Lipid mediators are important endogenous regulators of neural cell proliferation, differentiation, oxidative stress, inflammation, and apoptosis. They originate from enzymic degradation of glycerophospholipids, sphingolipids, and cholesterol by phospholipases, sphingomyelinases, and cytochrome P450 hydroxylases, respectively. Arachidonic acid-derived lipid mediators are called eicosanoids. Eicosanoids have emerged as key regulators of cell proliferation, differentiation, oxidative stress, and neuroinflammation. Another arachidonic acid-derived lipid mediator is lipoxin. Eicosanoids have proinflammatory effects, whereas lipoxins produce antiinflammatrory effects. The crossponding lipid mediators of docosahexaenoic acid metabolism are named docosanoids. They include resolvins, protectins, and neuroprotectins. Docosanoids produce antioxidant, anti-inflammatory, and antiapoptotic effects in the brain tissue. Other glycerophospholipid-derived lipid mediators are platelet-activating factor, lysophosphatidic acid, and endocannabinoids. Degradation of sphingolipids also results in the generation of sphingolipid-derived lipid mediators. Sphingolipid-derived lipid mediators are ceramide, ceramide 1-phosphate, sphingosine, and sphingosine 1-phosphate. They mediate cellular differentiation, cell growth, and apoptosis. Similarly, cholesterol-derived lipid mediators hydroxycholesterol and oxycholesterol produce apoptosis. Most of these mediators originate from the plasma membrane. The nucleus has its own set of enzymes and lipid mediators that originate from the nuclear envelope and matrix. The purpose of this commentary is to describe basic and clinical information on lipid mediators in the nucleus.

Direct interaction between the inhibitor 2 and ceramide via sphingolipid-protein binding is involved in the regulation of protein phosphatase 2A activity and signaling

In this study, the inhibitor 2 of protein phosphatase 2A (I2PP2A) was identified in vitro and in situ as a ceramide-binding protein, which exhibits stereoisomer specificity and fatty acid chain length preference. Site- directed mutagenesis coupled with structural details of I2PP2A suggested that VIK 207-209 residues localized on helix 7 are important for ceramide binding and single mutation of K209D altered this interaction. Notably, I2PP2A-ceramide binding decreased the association between PP2A and the inhibitor, preventing the inhibition of PP2A activity in vitro. In addition, studies in A549 human lung cancer cells revealed that ceramide mediates c-Myc degradation via its PP2A-dependent dephosphorylation at S62, and treatment with okadaic acid and expression of c-Myc mutants with S62A or S62D conversions resulted in resistance to ceramide-mediated degradation. Importantly, whereas down-regulation of I2PP2A enhanced PP2A-mediated c-Myc degradation in response to ceramide, ectopic expression of wild-type I2PP2A but not of its K209D mutant protected this degradation in A549 cells. Moreover, expression of wild-type I2PP2A prevented the growth-inhibitory effects of ceramide both against A549 cells and xenograft-driven tumors in situ and in vivo compared with that in controls. Thus, these results suggest that direct interaction of I2PP2A with ceramide plays important biological roles via the regulation of PP2A activity and signaling, which in turn control ceramide-mediated degradation of c-Myc and antiproliferation.

Lipid microdomains in cell nucleus

It is known that nuclear lipids play a role in proliferation, differentiation, and apoptotic process. Cellular nuclei contain high levels of phosphatidylcholine and sphingomyelin, which are partially linked with cholesterol and proteins to form lipid-protein complexes. These lipids are also associated with transcription factors and newly synthesized RNA but, up to date, their organization is still unknown. The aim of the present work was to study if these specific lipid-protein interactions could be nuclear membrane microdomains and to evaluate their possible role. The results obtained demonstrate for the first time the existence of nuclear microdomains characterized by a specific lipid composition similar to that of intranuclear lipid-protein complexes previously described. Nuclear microdomain lipid composition changes during cell proliferation when the content of newly synthesized RNA increases. Because previous data show a correlation between nuclear lipids and transcription process, the role of nuclear microdomains in cellular functions is discussed.

Ceramide signaling in cancer and stem cells

Most of the previous work on the sphingolipid ceramide has been devoted to its function as an apoptosis inducer. Recent studies, however, have shown that in stem cells, ceramide has additional nonapoptotic functions. In this article, ceramide signaling will be reviewed in light of 'systems interface biology': as an interconnection of sphingolipid metabolism, membrane biophysics and cell signaling. The focus will be on the metabolic interconversion of ceramide and sphingomyelin or sphingosine-1-phosphate. Lipid rafts and sphingolipid-induced protein scaffolds will be discussed as a membrane interface for lipid-controlled cell signaling. Ceramide/sphingomyelin and ceramide/sphingosine-1-phosphate-interdependent cell-signaling pathways are significant for the regulation of cell polarity, apoptosis and/or proliferation, and as novel pharmacologic targets in cancer and stem cells.

Phosphatidylcholine/sphingomyelin metabolism crosstalk inside the nucleus

It is known that phospholipids represent a minor component of chromatin. It has been highlighted recently that these lipids are metabolized directly inside the nucleus, thanks to the presence of enzymes related to their metabolism, such as neutral sphingomyelinase, sphingomyelin synthase, reverse sphingomyelin synthase and phosphatidylcholine-specific phospholipase C. The chromatin enzymatic activities change during cell proliferation, differentiation and/or apoptosis, independently from the enzyme activities present in nuclear membrane, microsomes or cell membranes. This present study aimed to investigate crosstalk in lipid metabolism in nuclear membrane and chromatin isolated from rat liver in vitro and in vivo. The effect of neutral sphingomyelinase activity on phosphatidylcholine-specific phospholipase C and sphingomyelin synthase, which enrich the intranuclear diacylglycerol pool, and the effect of phosphatidylcholine-specific phospholipase C activity on neutral sphingomyelinase and reverse sphingomyelin synthase, which enrich the intranuclear ceramide pool, was investigated. The results show that in chromatin, there exists a phosphatidylcholine/sphingomyelin metabolism crosstalk which regulates the intranuclear ceramide/diacylglycerol pool. The enzyme activities were inhibited by D609, which demonstrated the specificity of this crosstalk. Chromatin lipid metabolism is activated in vivo during cell proliferation, indicating that it could play a role in cell function. The possible mechanism of crosstalk is discussed here, with consideration to recent advances in the field.

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.

Smart drugs for smarter stem cells: making SENSe (sphingolipid-enhanced neural stem cells) of ceramide

Ceramide and its derivative sphingosine-1-phosphate (S1P) are important signaling sphingolipids for neural stem cell apoptosis and differentiation. Most recently, our group has shown that novel ceramide analogs can be used to eliminate teratoma (stem cell tumor)-forming cells from a neural stem cell graft. In new studies, we found that S1P promotes survival of specific neural precursor cells that undergo differentiation to cells expressing oligodendroglial markers. Our studies suggest that a combination of novel ceramide and S1P analogs eliminates tumor-forming stem cells and at the same time, triggers oligodendroglial differentiation. This review discusses recent studies on the function of ceramide and S1P for the regulation of apoptosis, differentiation, and polarity in stem cells. We will also discuss results from ongoing studies in our laboratory on the use of sphingolipids in stem cell therapy.

(Glyco)sphingolipidology: an amazing challenge and opportunity for systems biology

Sphingolipids are found in essentially all eukaryotes and in some prokaryotes and viruses, where they influence cell structure, signaling and interactions with the extracellular environment. Because of the combinatorial nature of their biosynthesis, the sphingolipidome comprises untold thousands of species that encompass bioactive backbones and complex phospho- and glycolipids. Mass spectrometry is able to analyze a growing fraction of the sphingolipidome and is beginning to provide information about localization. Use of these structure specific, quantitative methods is producing insights, and surprises, regarding sphingolipid structure, metabolism, function and disease. Dealing with such large data sets poses special challenges for systems biology, but the intrinsic and elegant interrelationships among these compounds might provide a key to dealing with the complexity of the sphingolipidome.

GM1 in the nuclear envelope regulates nuclear calcium through association with a nuclear sodium-calcium exchanger

The inner membrane of the nuclear envelope (NE) of neurons and other cells has been shown to contain GM1 tightly associated with a Na(+)/Ca(2+) exchanger (NCX) whose activity it potentiates in mediating transfer of Ca(2+) from nucleoplasm to the NE lumen. This is consistent with localization of the NCX/GM1 complex in the inner membrane of the NE. NCXs of the plasma membrane, in contrast, appear to bind GM1 much less avidly. This is believed due to different isoforms of NCX in the two membranes, and a difference in topology of NCX relative to GM1. A cytoprotective function for nuclear NCX/GM1 was suggested in the observation that cultured neurons from mice lacking GM1 (GM2/GD2 synthase knockout) were vulnerable to Ca(2+)-induced apoptosis. These neurons in culture were rescued to some extent by GM1 but more effectively by LIGA-20, a membrane-permeant derivative of GM1 that entered the NE. Further indication came from a study of the mutant mice, which were highly susceptible to kainate-induced seizures and could be rescued by LIGA-20. This correlated with the ability of LIGA-20 to cross the blood-brain barrier, enter brain cells, insert into the NE, and potentiate nuclear NCX.

Multiple roles for sphingolipids in steroid hormone biosynthesis

Steroid hormones are essential regulators of a vast number of physiological processes. The biosynthesis of these chemical messengers occurs in specialized steroidogenic tissues via a multi-step process that is catalyzed by members of the cytochrome P450 superfamily of monooxygenases and hydroxysteroid dehydrogenases. Though numerous signaling mediators, including cytokines and growth factors control steroidogenesis, trophic peptide hormones are the primary regulators of steroid hormone production. These peptide hormones activate a cAMP/cAMP-dependent kinase (PKA) signaling pathway, however, studies have shown that crosstalk between multiple signal transduction pathways and signaling molecules modulates optimal steroidogenic capacity. Sphingolipids such as ceramide, sphingosine, sphingosine-1-phosphate, sphingomyelin, and gangliosides have been shown to control the steroid hormone biosynthetic pathway at multiple levels, including regulating steroidogenic gene expression and activity as well as acting as second messengers in signaling cascades. In this review, we provide an overview of recent studies that have investigated the role of sphingolipids in adrenal, gonadal, and neural steroidogenesis.

Cyclic AMP-stimulated interaction between steroidogenic factor 1 and diacylglycerol kinase theta facilitates induction of CYP17

In the human adrenal cortex, adrenocorticotropin (ACTH) activates CYP17 transcription by promoting the binding of the nuclear receptor steroidogenic factor 1 (SF1) (Ad4BP, NR5A1) to the promoter. We recently found that sphingosine is an antagonist for SF1 and inhibits cyclic AMP (cAMP)-dependent CYP17 gene transcription. The aim of the current study was to identify phospholipids that bind to SF1 and to characterize the mechanism by which ACTH/cAMP regulates the biosynthesis of this molecule(s). Using tandem mass spectrometry, we show that in H295R human adrenocortical cells, SF1 is bound to phosphatidic acid (PA). Activation of the ACTH/cAMP signal transduction cascade rapidly increases nuclear diacylglycerol kinase (DGK) activity and PA production. PA stimulates SF1-dependent transcription of CYP17 reporter plasmids, promotes coactivator recruitment, and induces the mRNA expression of CYP17 and several other steroidogenic genes. Inhibition of DGK activity attenuates the binding of SF1 to the CYP17 promoter, and silencing of DGK-theta expression inhibits cAMP-dependent CYP17 transcription. LXXLL motifs in DGK-theta mediate a direct interaction of SF1 with the kinase and may facilitate binding of PA to the receptor. We conclude that ACTH/cAMP stimulates PA production in the nucleus of H295R cells and that this increase in PA concentrations facilitates CYP17 induction.

Sodium-calcium exchangers in the nucleus: an unexpected locus and an unusual regulatory mechanism

Whereas sodium-calcium exchangers (NCXs) have long been recognized as plasma membrane constituents that serve to maintain homeostatic concentrations of Ca2+ in the cytoplasm, they were recently shown to also occur in the nuclear envelope (NE) of neural and other cells where they function to regulate nuclear Ca2+. A unique feature of NCXs in the NE is their high-affinity binding to GM1 ganglioside, this association being required for optimal exchanger activity. The NCX-GM1 complex occurs in the inner membrane of the NE and transfers Ca2+ from the nucleoplasm to the NE lumen. In neuronal cells, nuclear GM1 levels are low prior to differentiation but increase rapidly as axonal outgrowth progresses. Cells from genetically altered mice lacking GM1 have limited ability to regulate nuclear Ca2+, and the mice themselves showed similar deficit as seen in their high susceptibility to kainite-induced seizures. These are attenuated by LIGA-20, a derivative of GM1 that enters the nuclear membrane and restores nuclear NCX activity to normal level.

Short-term manipulation of plasma free fatty acids does not change skeletal muscle concentrations of ceramide and glucosylceramide in lean and overweight subjects

CONTEXT

Increased plasma free fatty acid (FFA) concentrations may be in part responsible for the increased levels of ceramide in skeletal muscle of obese subjects.

OBJECTIVE

We studied the effect of lowering and increasing plasma FFA levels on muscle ceramide and glucosylceramide concentrations in lean and obese subjects.

DESIGN

Plasma FFAs were either increased or decreased for 6 h by infusing a lipid emulsion or using Acipimox, respectively. Muscle biopsies were performed before and after the intervention for measurements of ceramide and glucosylceramide.

STUDY SUBJECTS

Eight lean [body mass index 21.9 (range, 19.6-24.6) kg/m2] and six overweight/obese [body mass index 34.4 (27.8-42.5) kg/m2] subjects without type 2 diabetes mellitus participated in the study.

MAIN OUTCOME MEASURE

Differences in muscle ceramide and glucosylceramide upon manipulation of plasma FFAs were measured.

RESULTS

There were no differences in muscle ceramide and glucosylceramide between lean and obese subjects, respectively. Increasing or decreasing plasma FFAs for 6 h had no effect on ceramide [high FFAs: 24 (19-25) vs. 24 (22-27) pmol/mg muscle, P=0.46; and 22 (20-28) vs. 24 (18-26) pmol/mg muscle, P=0.89 in lean and obese, respectively; low FFAs: 26 (24-35) vs. 23 (18-27) pmol/mg muscle, P=0.17 and 24 (15-44) vs. 24 (19-42) pmol/mg muscle, P=0.6 in lean and obese, respectively] and glucosylceramide [high FFAs: 2.0 (1.7-4.3) vs. 3.4 (2.1-4.6) pmol/mg muscle, P=0.17; and 3.0 (1.3-6.7) vs. 2.6 (1.2-3.9) pmol/mg muscle, P=0.89 in lean and obese, respectively; low FFAs: 2.2 (1.0-4.4) vs. 1.7 (1.4-3.0) pmol/mg muscle, P=0.92; and 6.6 (1.0-25.0) vs. 4.3 (1.3-7.6) pmol/mg muscle, P=0.7 in lean and obese, respectively] concentrations in skeletal muscle.

CONCLUSION

Short-term manipulation of plasma FFAs has no effect on ceramide and glucosylceramide concentrations in skeletal muscle from lean and obese subjects.

Glycolipid transfer proteins

Glycolipid transfer proteins (GLTPs) are small (24 kDa), soluble, ubiquitous proteins characterized by their ability to accelerate the intermembrane transfer of glycolipids in vitro. GLTP specificity encompasses both sphingoid- and glycerol-based glycolipids, but with a strict requirement that the initial sugar residue be beta-linked to the hydrophobic lipid backbone. The 3D architecture of GLTP reveals liganded structures with unique lipid-binding modes. The biochemical properties of GLTP action at the membrane surface have been studied rather comprehensively, but the biological role of GLTP remains enigmatic. What is clear is that GLTP differs distinctly from other known glycolipid-binding proteins, such as nonspecific lipid transfer proteins, lysosomal sphingolipid activator proteins, lectins, lung surfactant proteins as well as other lipid-binding/transfer proteins. Based on the unique conformational architecture that targets GLTP to membranes and enables glycolipid binding, GLTP is now considered the prototypical and founding member of a new protein superfamily in eukaryotes.

Diverse pathways for nuclear signaling by G protein‐coupled receptors and their ligands

Recent realization that plasma membrane G protein-coupled receptors (GPCRs) may translocate and establish ligand-responsive signaling complexes in other cellular structures has motivated studies of site-specific differences in transductional pathways. GPCRs and their ligands may signal transcription and other nuclear events by two basic mechanisms. The first consists of GPCR-complex activation of messengers that enter the nucleus and there initiate cell-modifying processes without the GPCR leaving the plasma membrane. The second encompasses entry into the nuclear membranes or matrix of either GPCR ligands, which bind to non-GPCR nuclear signaling proteins, proteolytic fragments of GPCRs capable of ligand-independent signaling, or intact GPCRs with transduction-competent factors that directly initiate or regulate transcriptional events. With the second mechanism, often concurrent down-regulation of plasma membrane GPCRs terminates signaling from the cell-surface and moves it into the nuclear domain. Site-dependent differences in signals from the same GPCR provide potentials for unique cellular abnormalities attributable to defective intracellular movement and distribution of a GPCR, site-specific alterations in ligand concentration, and limited intracellular bioavailability of pharmacological agents that can interact specifically with both nuclear and plasma membrane forms of a GPCR.

Distinctive T cell-suppressive signals from nuclearized type 1 sphingosine 1-phosphate G protein-coupled receptors

Sphingosine 1-phosphate (S1P) generated by cells of innate immunity and the type 1 S1P G protein-coupled receptor (S1P(1)) on mobile T cells constitute a major system for control of lymphoid organ traffic and tissue migration of T cells. Now we show that T cell activation mediated by the T cell antigen receptor translocates plasma membrane S1P(1) to nuclear envelope membranes for association there with G(i/o), Erk (1/2), and other proteins that plasma membrane S1P(1) uses to signal T cell proliferation. However, nuclear S1P(1) and plasma membrane S1P(1) transduce opposite effects of S1P on T cell proliferation and relevant signaling as exemplified by respective decreases and increases in T cell nuclear concentrations of both phospho-Erk and active (phosphorylated) c-Jun. T cell antigen receptor-mediated activation of T cells therefore both eliminates migration responses to S1P by down-regulation of plasma membrane S1P(1) and translocates the S1P-S1P(1) axis into the nuclear domain where signals are directed to transcriptional control of immune functions other than migration.

Thematic review series: systems biology approaches to metabolic and cardiovascular disorders. Lipidomics: a global approach to lipid analysis in biological systems

Lipids are water-insoluble molecules that have a wide variety of functions within cells, including: 1) maintenance of electrochemical gradients; 2) subcellular partitioning; 3) first- and second-messenger cell signaling; 4) energy storage; and 5) protein trafficking and membrane anchoring. The physiological importance of lipids is illustrated by the numerous diseases to which lipid abnormalities contribute, including atherosclerosis, diabetes, obesity, and Alzheimer's disease. Lipidomics, a branch of metabolomics, is a systems-based study of all lipids, the molecules with which they interact, and their function within the cell. Recent advances in soft-ionization mass spectrometry, combined with established separation techniques, have allowed the rapid and sensitive detection of a variety of lipid species with minimal sample preparation. A "lipid profile" from a crude lipid extract is a mass spectrum of the composition and abundance of the lipids it contains, which can be used to monitor changes over time and in response to particular stimuli. Lipidomics, integrated with genomics, proteomics, and metabolomics, will contribute toward understanding how lipids function in a biological system and will provide a powerful tool for elucidating the mechanism of lipid-based disease, for biomarker screening, and for monitoring pharmacologic therapy.