Formation of endogenous free sphingoid bases in cells induced by changing medium conditions

Quantification of sphingosine and sphinganine from crude lipid extracts by HPLC electrospray ionization tandem mass spectrometry

Sphingosine (SPH) comprises the backbone of sphingolipids and is known as a second messenger involved in the modulation of cell growth, differentiation, and apoptosis. The currently available methods for the quantification of SPH are, in part, complicated, time-consuming, insensitive, or unselective. Therefore, a fast and convenient methodology for the quantification of SPH and the biosynthetic intermediate sphinganine (SPA) was developed. The method is based on an HPLC separation coupled to electrospray ionization tandem mass spectrometry (MS/MS). Quantitation is achieved by the use of a constant concentration of a non-naturally occurring internal standard, 17-carbon chain SPH (C17-SPH), together with a calibration curve established by spiking different concentrations of naturally occurring sphingoid bases. SPH and SPA coeluted with C17-SPH, which allows an accurate correction of the analyte response. Interference of the SPH+2 isotope with SPA quantification was corrected by an experimentally determined factor. The limits of detection were 9 fmol for SPH and 21 fmol for SPA. The overall coefficients of variation were 8% and 13% for SPH and SPA, respectively. The developed HPLC-tandem mass spectrometry methodology, with an analysis time of 3.5 min, simple sample preparation, and automated data analysis, allows high-throughput quantification of sphingoid bases from crude lipid extracts and is a valuable tool for studies of cellular sphingolipid metabolism and signaling.

Serine palmitoyltransferase, a key enzyme of sphingolipid metabolism

The first step in the biosynthesis of sphingolipids is the condensation of serine and palmitoyl CoA, a reaction catalyzed by serine palmitoyltransferase (SPT) to produce 3-ketodihydrosphingosine (KDS). This review focuses on recent advances in the biochemistry and molecular biology of SPT. SPT belongs to a family of pyridoxal 5'-phosphate (PLP)-dependent alpha-oxoamine synthases (POAS). Mammalian SPT is a heterodimer of 53-kDa LCB1 and 63-kDa LCB2 subunits, both of which are bound to the endoplasmic reticulum (ER) most likely with the type I topology, whereas other members of the POAS family are soluble homodimer enzymes. LCB2 appears to be unstable unless it is associated with LCB1. Potent inhibitors of SPT structurally resemble an intermediate in a probable multistep reaction mechanism for SPT. Although SPT is a housekeeping enzyme, its activity is regulated transcriptionally and post-transcriptionally, and its up-regulation is suggested to play a role in apoptosis induced by certain types of stress. Specific missense mutations in the human LCB1 gene cause hereditary sensory neuropathy type I, an autosomal dominantly inherited disease, and these mutations confer dominant-negative effects on SPT activity.

Identification of ammonium ion and 2,6-bis(omega-aminobutyl)- 3, 5-diiminopiperazine as endogenous factors that account for the "burst" of sphingosine upon changing the medium of J774 cells in culture

Cells in culture often undergo a "burst" of free sphingosine, sphingosine 1-phosphate, ceramide, and other bioactive lipids upon removal of "conditioned" medium, and at least one lipid signaling pathway (protein kinase C) has been shown to be affected by these changes (Smith, E. R. & Merrill A. H., Jr. (1995) J. Biol. Chem. 270, 18749-18758; Smith, E. R., Jones, P. L., Boss, J. M. & Merrill, A. H., Jr. (1997) J. Biol. Chem. 272, 5640-5646). Whereas increases in sphinganine and dihydroceramide are responses to provision of precursors for sphingolipid biosynthesis de novo in the new medium, the sphingosine burst is due to sphingolipid turnover upon removal of suppressive factor(s) in conditioned medium. This study describes the purification and characterization of these suppressive factors. Conditioned medium from J774 cells was fractionated into two components that suppress the burst as follows: ammonium ion, which reaches 2-3 mM within 48 h of cell culture; and a low molecular weight, cationic compound that has been assigned the structure 2, 6-bis(omega-aminobutyl)-3,5-diimino-piperazine (for which we suggest the name "batrachamine" based on its appearance) by (1)H and (13)C NMR, Fourier transform infrared spectroscopy, and mass spectrometric analyses. The physiological significance of these compounds as suppressors of sphingolipid metabolism is unclear; however, ammonium ion is a by-product of amino acid catabolism and reaches high concentrations in some tissues. Batrachamine is even more intriguing because this is, as far as we are aware, the first report of a naturally occurring compound of this structural type. Considering the many cell functions that are affected by sphingoid bases and their derivatives, the effects of NH(4) and batrachamine on sphingolipid metabolism may have important implications for cell regulation.

Activation of human Valpha24NKT cells by alpha-glycosylceramide in a CD1d-restricted and Valpha24TCR-mediated manner

Vact14NK(natural killer) T cells play an important role in controlling tumors or in preventing autoimmunity in the murine system. Valpha24NKT cells, the human counterpart of Valpha14NKT cells, may contribute to controlling the progression of autoimmune diseases in humans. These findings show the possibility that ligand(s) for these NKT cells can control the above-mentioned pathological conditions. Specific glycolipids such as alpha-galactosylceramide (alpha-GalCer) and alpha-glucosylceramide (alpha-GlcCer) have been identified as ligand(s) recognized by murine Valpha14NKT cells in a CD1d-restricted manner, but it remains unclear whether these glycolipids are ligand(s) for Valpha24NKT cells in humans. To determine whether alpha-glycosylceramide is presented by CD1d molecules in humans, we initially established a Valpha24NKT cell line specific for alpha-glycosylceramide using dendritic cell (DC) like cells from normal peripheral blood mononuclear cells (PBMC) in an autologous mixed leukocyte reaction (auto-MLR) system, and characterized the Valpha24NKT cell line. The Valpha24NKT cells were CD3+ CD4-CD8-Valpha24+Vbeta11+NKRP1A+ and specifically proliferated in response to alpha-glycosylceramide in CD1d-restricted and Valpha24TCR-mediated manner. The phenotypic and functional similarities between murine Valpha14NKT cells and human Valpha24NKT cells suggest that Valpha24NKT cells may play an important role in controlling tumors or in preventing autoimmunity as observed with Valpha14NKT cells.

A novel sphingosine-dependent protein kinase (SDK1) specifically phosphorylates certain isoforms of 14-3-3 protein

Protein kinases activated by sphingosine or N,N'-dimethylsphingosine, but not by other lipids, have been detected and are termed sphingosine-dependent protein kinases (SDKs). These SDKs were previously shown to phosphorylate endogenous 14-3-3 proteins (Megidish, T., White, T., Takio, K., Titani, K., Igarashi, Y., and Hakomori, S. (1995) Biochem. Biophys. Res. Commun. 216, 739-747). We have now partially purified one SDK, termed SDK1, from cytosol of mouse Balb/c 3T3(A31) fibroblasts. SDK1 is a serine kinase with molecular mass 50-60 kDa that is strongly activated by N, N'-dimethylsphingosine and sphingosine, but not by ceramide, sphingosine 1-phosphate, or other sphingo-, phospho-, or glycerolipids tested. Its activity is inhibited by the protein kinase C activator phosphatidylserine. Activity of SDK1 is clearly distinct from other types of serine kinases tested, including casein kinase II, the alpha and zeta isoforms of protein kinase C, extracellular signal-regulated mitogene-activated protein kinase 1 (Erk-1), Erk-2, and Raf-1. SDK1 specifically phosphorylates certain isoforms of 14-3-3 (eta, beta, zeta) but not others (sigma, tau). The phosphorylation site was identified as Ser* in the sequence Arg-Arg-Ser-Ser*-Trp-Arg in 14-3-3 beta. The sigma and tau isoforms of 14-3-3 lack serine at this position, potentially explaining their lack of phosphorylation by SDK1. Interestingly, the phosphorylation site is located on the dimer interface of 14-3-3. Phosphorylation of this site by SDK1 was studied in 14-3-3 mutants. Mutation of a lysine residue, located 9 amino acids N-terminal to the phosphorylation site, abolished 14-3-3 phosphorylation. Furthermore, co-immunoprecipitation experiments demonstrate an association between an SDK and 14-3-3 in situ. Exogenous N, N'-dimethylsphingosine stimulates 14-3-3 phosphorylation in Balb/c 3T3 fibroblasts, suggesting that SDK1 may phosphorylate 14-3-3 in situ. These data support a biological role of SDK1 activation and consequent phosphorylation of specific 14-3-3 isoforms that regulate signal transduction. In view of the three-dimensional structure of 14-3-3, it is likely that phosphorylation by SDK1 would alter dimerization of 14-3-3, and/or induce conformational changes that alter 14-3-3 association with other kinases involved in signal transduction.

Dual actions of sphingosine-1-phosphate: extracellular through the Gi-coupled receptor Edg-1 and intracellular to regulate proliferation and survival

Sphingosine-1-phosphate (SPP), a bioactive lipid, acts both intracellularly and extracellularly to cause pleiotropic biological responses. Recently, we identified SPP as a ligand for the G protein-coupled receptor Edg-1 (Lee, M.-J., J.R. Van Brocklyn, S. Thangada, C.H. Liu, A.R. Hand, R. Menzeleev, S. Spiegel, and T. Hla. 1998. Science. 279:1552-1555). Edg-1 binds SPP with remarkable specificity as only sphinganine-1-phosphate displaced radiolabeled SPP, while other sphingolipids did not. Binding of SPP to Edg-1 resulted in inhibition of forskolin-stimulated cAMP accumulation, in a pertussis toxin-sensitive manner. In contrast, two well-characterized biological responses of SPP, mitogenesis and prevention of apoptosis, were clearly unrelated to binding to Edg-1 and correlated with intracellular uptake. SPP also stimulated signal transduction pathways, including calcium mobilization, activation of phospholipase D, and tyrosine phosphorylation of p125(FAK), independently of edg-1 expression. Moreover, DNA synthesis in Swiss 3T3 fibroblasts was significantly and specifically increased by microinjection of SPP. Finally, SPP suppresses apoptosis of HL-60 and pheochromocytoma PC12 cells, which do not have specific SPP binding or expression of Edg-1 mRNA. Conversely, sphinganine-1-phosphate, which binds to and signals via Edg-1, does not have any significant cytoprotective effect. Thus, SPP is a prototype for a novel class of lipid mediators that act both extracellularly as ligands for cell surface receptors and intracellularly as second messengers.

The potent lipid mitogen sphingosylphosphocholine activates the DNA binding activity of upstream stimulating factor (USF), a basic helix-loop-helix-zipper protein

We previously demonstrated that the sphingolipid, sphingosylphosphocholine (SPC) increased DNA binding activity of AP-1 proteins accompanying cellular proliferation. Herein, the effects of SPC on DNA binding activity and transcription of the basic, helix-loop-helix, leucine zipper (bHLH-ZIP) proteins Myc, Max, and USF were investigated because they regulate genes involved in mitogenesis. E-box (CACGTG) DNA binding proteins were detected by electrophoretic mobility shift assays in nuclear extracts from Swiss 3T3 fibroblasts. The slowest migrating complex (complex I) increased within 1-3 min after treatment with SPC, remained elevated for 10 min, and increased again after 12 h. Complexes I and II contained USF-1 and USF-2 proteins, and complex I migrated similarly to recombinant USF-1 protein/DNA complex. Treatment of nuclear extracts with alkaline phosphatase decreased these complexes suggesting USF might be a phosphoprotein, post-translationally modified by SPC. max and usf-1 mRNA levels were unaffected by SPC treatment. In contrast, c-myc mRNA was rapidly elevated, reached maximum levels at 0.5-1 h, and showed an additional increase after 12 h, just preceding S phase. Thus, certain bHLH-ZIP transcription factors may be involved in cell growth regulation by SPC.

CD1d-restricted and TCR-mediated activation of valpha14 NKT cells by glycosylceramides

Natural killer T (NKT) lymphocytes express an invariant T cell antigen receptor (TCR) encoded by the Valpha14 and Jalpha281 gene segments. A glycosylceramide-containing alpha-anomeric sugar with a longer fatty acyl chain (C26) and sphingosine base (C18) was identified as a ligand for this TCR. Glycosylceramide-mediated proliferative responses of Valpha14 NKT cells were abrogated by treatment with chloroquine-concanamycin A or by monoclonal antibodies against CD1d/Vbeta8, CD40/CD40L, or B7/CTLA-4/CD28, but not by interference with the function of a transporter-associated protein. Thus, this lymphocyte shares distinct recognition systems with either T or NK cells.

Changing J774A.1 cells to new medium perturbs multiple signaling pathways, including the modulation of protein kinase C by endogenous sphingoid bases

Sphingosine, sphinganine, and other long-chain (sphingoid) bases are highly bioactive intermediates of sphingolipid metabolism that have diverse effects when added to cells, including the inhibition of protein kinase C (PKC) as evaluated by both enzymatic activity and [3H]phorbol dibutyrate ([3H]PDBu) binding. Nonetheless, changes in endogenous sphingoid bases have not been proven to affect PKC or other signal transduction pathways. We have discovered recently that changing J774A.1 cells to new medium results in up to 10-fold increases in sphingoid bases (Smith, E. R., and Merrill, A. H., Jr. (1995) J. Biol. Chem. 270, 18749-18758); therefore, this system was used to elevate sphingosine and sphinganine and determine if PKC was affected. Incubation of J774A.1 cells in new medium for 30 min increased the levels of these endogenous sphingoid bases to approximately 0.5 nmol/mg of protein and decreased [3H]PDBu binding by 40-60%. Addition of NH4Cl, which suppresses the change in sphingosine, restored [3H]PDBu binding. Elevation of endogenous sphinganine by a second method (addition of fumonisin B1, an inhibitor of ceramide synthase) also reduced [3H]PDBu binding; therefore, elevations in sphingosine and sphinganine can both affect PKC. The elevation in sphingoid bases was also associated with an increase in the amount of PKC-delta (the major PKC isozyme in J774A. 1 cells) in the cytosol, as determined by activity assays and immunoblot analyses. Changing the culture medium affected other PKC isozymes, increased cellular levels of diacylglycerol, dihydroceramide, and ceramide, and altered the expression of two genes (the expression of JE was increased, and the induction of MnSOD by TNF-alpha was potentiated). Thus, changing the culture medium has numerous effects on J774A.1 cells, including the modulation of PKC by endogenous sphingoid bases.

Involvement of sphingolipids metabolites in cellular proliferation modulated by ganglioside GM1

The B subunit of cholera toxin, which binds specifically to ganglioside GM1, is mitogenic for quiescent Swiss 3T3 fibroblasts. Recently, sphingolipids metabolites, ceramide, sphingosine and sphingosine-1-phosphate, have been implicated as second messengers in cell growth regulation and differentiation. In this paper, we examined the possibility that interaction of the B subunit with membrane GM1 leads to alterations in metabolism of glycosphingolipids and that increased levels of sphingolipids metabolites may mediate the biological effects of the B subunit. While the B subunit did not induce a change in the level of ceramide or sphingosine, the level of sphingosine-1-phosphate was rapidly and transiently increased. The B subunit also transiently activated cytosolic sphingosine kinase activity, which catalyzes the phosphorylation of the primary hydroxyl group of sphingosine to produce sphingosine-1-phosphate. To determine whether the increase in sphingosine-1-phosphate level plays a role in B subunit-induced mitogenicity, we used a competitive inhibitor of sphingosine kinase, D,L-threo-dihydrosphingosine. D,L-thereo-Dihydrosphingosine not only inhibited B subunit-induced DNA synthesis by 26%, it also reduced its ability to stimulate DNA-binding activity of the transcription factor AP-1. This sphingosine kinase inhibitor also inhibited B subunit-induced increases in the activity of cell cycle-regulated, cyclin-dependent serine/threonine kinases, cdk2 and p34cdc2. These findings suggest that sphingosine-1-phosphate may play a role in the signal transduction pathways activated by binding of the B subunit to endogenous ganglioside GM1.

Sphingomyelin metabolism is linked to salt transport in the gills of euryhaline fish

By in vivo and in vitro studies of L-(3-3H)serine and [9,10(n)-3H]palmitic acid incorporation into phospholipids, we show a change in the renewal of the ceramide moiety of sphingomyelin in the gills of euryhaline fish (sea bass and eels) when the animals were subjected to abrupt alterations in environmental salinity. In vivo, decrease of the salinity from sea water (salinity 3.7%) to diluted sea water (salinity 1%) induced an increase of label incorporation into gill sphingomyelin. The same was true when gills from sea water-adapted sea bass or sea water-adapted eels were incubated in diluted sea water. A decrease in free ceramides synthesis was also observed in the gills of sea water-adapted sea bass when the salinity of the incubation medium was reduced. Direct inhibition of Na+/K(+)-ATPase activity with ouabain decreased the sphingomyelin synthesis in the gills of sea bass during in vitro incubation in diluted sea water, whereas treatment with furosemide stimulated sphingomyelin synthesis in the same gills incubated in sea water. These findings indicate that changes in Na+ fluxes modify the sphingomyelin turnover and control the production of free ceramides and sphingosine in gill cells of euryhaline fish. In view of the well-known effects of these sphingomyelin degradation products on isolated tumor cell differentiation, we suggest that they play a very important role in modulating chloride cell distribution and metabolism of fish gills during abrupt changes in environmental salinity.

Differential roles of de novo sphingolipid biosynthesis and turnover in the "burst" of free sphingosine and sphinganine, and their 1-phosphates and N-acyl-derivatives, that occurs upon changing the medium of cells in culture

Long-chain (sphingoid) bases are highly bioactive intermediates for sphingolipid metabolism, yet relatively little is known about how the amounts of these compounds are regulated. This study used J774A.1 cells to characterize the "burst" of sphinganine and sphingosine, or the transient increase of up to 10-fold in long-chain base mass, that occurs when cells in culture are changed to fresh medium. The increase in sphinganine was attributable to de novo sphingolipid biosynthesis because: 1) there is increased incorporation of [3H]serine and [3H]palmitate into sphinganine; 2) the incorporation of [3H]serine was equivalent to the increase in sphinganine mass; 3) beta-F-alanine, an inhibitor of serine palmitoyltransferase, blocked the sphinganine burst; 4) the magnitude of the burst depended on the concentration of serine in the medium, which is known to affect long-chain base biosynthesis; and 5) the appearance of sphinganine was relatively unaffected by lyso-osmotrophic agents (NH4Cl and chloroquine) that blocked sphingolipid hydrolysis in these cells. In contrast, the sphingosine burst arose mainly from turnover of complex sphingolipids because no incorporation of [3H]serine or [3H]palmitate into sphingosine was detected; sphingosine mass was not affected by beta-F-alanine or the serine concentration; and, the burst could be followed by the release of sphingosine and ceramide from complex sphingolipids (especially sphingomyelin) in a process that was inhibited by NH4Cl and chloroquine. Additionally, the fate of these long-chain bases differed: sphinganine was mostly (80-85%) acylated and incorporated into dihydroceramide and complex sphingolipids, whereas most of the sphingosine (70%) was phosphorylated and degraded, with incorporation of the resulting ethanolamine phosphate into phosphatidylethanolamine. Sphinganine, however, could be diverted toward degradation by adding an inhibitor of N-acylation (fumonisin B1). In accounting for the elevation in sphingosine and sphinganine after cells are changed to new medium, these studies have provided fundamental information about long-chain base metabolism. The existence of differential changes in sphinganine and sphingosine, as well as their 1-phosphates and N-acyl-derivatives, should be considered when evaluating the roles of sphingolipid metabolites in cell regulation.