A specific enhancing effect of N,N-dimethylsphingosine on epidermal growth factor receptor autophosphorylation. Demonstration of its endogenous occurrence (and the virtual absence of unsubstituted sphingosine) in human epidermoid carcinoma A431 cells

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Sphingoid Bases Regulate the Sigma-1 Receptor-Sphingosine and N,N'-Dimethylsphingosine Are Endogenous Agonists

Both bioactive sphingolipids and Sigma-1 receptor (S1R) chaperones occur ubiquitously in mammalian cell membranes. Endogenous compounds that regulate the S1R are important for controlling S1R responses to cellular stress. Herein, we interrogated the S1R in intact Retinal Pigment Epithelial cells (ARPE-19) with the bioactive sphingoid base, sphingosine (SPH), or the pain-provoking dimethylated SPH derivative, N,N'-dimethylsphingosine (DMS). As informed by a modified native gel approach, the basal and antagonist (BD-1047)-stabilized S1R oligomers dissociated to protomeric forms in the presence of SPH or DMS (PRE-084 as control). We, thus, posited that SPH and DMS are endogenous S1R agonists. Consistently, in silico docking of SPH and DMS to the S1R protomer showed strong associations with Asp126 and Glu172 in the cupin beta barrel and extensive van der Waals interactions of the C18 alkyl chains with the binding site including residues in helices 4 and 5. Mean docking free energies were 8.73-8.93 kcal/mol for SPH and 8.56-8.15 kcal/mol for DMS, and calculated binding constants were ~40 nM for SPH and ~120 nM for DMS. We hypothesize that SPH, DMS, and similar sphingoid bases access the S1R beta barrel via a membrane bilayer pathway. We further propose that the enzymatic control of ceramide concentrations in intracellular membranes as the primary sources of SPH dictates availability of endogenous SPH and DMS to the S1R and the subsequent control of S1R activity within the same cell and/or in cellular environments.

How Dysregulated Ion Channels and Transporters Take a Hand in Esophageal, Liver, and Colorectal Cancer

Over the last two decades, the understanding of how dysregulated ion channels and transporters are involved in carcinogenesis and tumor growth and progression, including invasiveness and metastasis, has been increasing exponentially. The present review specifies virtually all ion channels and transporters whose faulty expression or regulation contributes to esophageal, hepatocellular, and colorectal cancer. The variety reaches from Ca²⁺, K⁺, Na⁺, and Cl^- channels over divalent metal transporters, Na⁺ or Cl^- coupled Ca²⁺, HCO₃^- and H⁺ exchangers to monocarboxylate carriers and organic anion and cation transporters. In several cases, the underlying mechanisms by which these ion channels/transporters are interwoven with malignancies have been fully or at least partially unveiled. Ca²⁺, Akt/NF-κB, and Ca²⁺- or pH-dependent Wnt/β-catenin signaling emerge as cross points through which ion channels/transporters interfere with gene expression, modulate cell proliferation, trigger epithelial-to-mesenchymal transition, and promote cell motility and metastasis. Also miRs, lncRNAs, and DNA methylation represent potential links between the misexpression of genes encoding for ion channels/transporters, their malfunctioning, and cancer. The knowledge of all these molecular interactions has provided the basis for therapeutic strategies and approaches, some of which will be broached in this review.

Dimethylsphingosine and miltefosine induce apoptosis in lung adenocarcinoma A549 cells in a synergistic manner

Lung cancer is one of the most common and lethal types of oncological diseases. Despite the advanced therapeutic approaches, the prognosis for lung cancer still remains poor. Apparently, there is an imperative need for more efficient therapeutic strategies. In this work we report that concurrent treatment of human adenocarcinoma A549 cells with specific concentrations of two antitumor agents, the sphingosine kinase 1 inhibitor N, N dimethylsphingosine (DMS) and the alkylphosphocholine miltefosine, induced synergistic cytotoxic effect, which was confirmed by calculation of the combination index. The simultaneous action of these agents, induced significant decrease of A549 cell number, as well as pronounced morphological alterations. Combined drugs caused substantial apoptotic events, and significant reduction of the pro-survival marker sphingosine- 1-phosphate (S1P), when compared to the individual treatments with each of the anticancer drugs alone. Miltefosine is known to affect the synthesis of choline-containing phospholipids, including sphingomyelin, but we report for the first time that it also reduces S1P. Here we suggest a putative mechanism underlying the effect of miltefosine on sphingosine kinase 1, involving miltefosine-induced inhibition of protein kinase C. In conclusion, our findings provide a possibility for treatment of lung cancer cells with lower concentrations of the two antitumor drugs, DMS and miltefosine, which is favorable, regarding their potential cytotoxicity to normal cells.

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.

Profiling the Essential Nature of Lipid Metabolism in Asexual Blood and Gametocyte Stages of Plasmodium falciparum

During its life cycle, Plasmodium falciparum undergoes rapid proliferation fueled by de novo synthesis and acquisition of host cell lipids. Consistent with this essential role, Plasmodium lipid synthesis enzymes are emerging as potential drug targets. To explore their broader potential for therapeutic interventions, we assayed the global lipid landscape during P. falciparum sexual and asexual blood stage (ABS) development. Using liquid chromatography-mass spectrometry, we analyzed 304 lipids constituting 24 classes in ABS parasites, infected red blood cell (RBC)-derived microvesicles, gametocytes, and uninfected RBCs. Ten lipid classes were previously uncharacterized in P. falciparum, and 70%-75% of the lipid classes exhibited changes in abundance during ABS and gametocyte development. Utilizing compounds that target lipid metabolism, we affirmed the essentiality of major classes, including triacylglycerols. These studies highlight the interplay between host and parasite lipid metabolism and provide a comprehensive analysis of P. falciparum lipids with candidate pathways for drug discovery efforts.

Inhibition of dihydroceramide desaturase activity by the sphingosine kinase inhibitor SKI II

Sphingosine kinase inhibitor (SKI) II has been reported as a dual inhibitor of sphingosine kinases (SKs) 1 and 2 and has been extensively used to prove the involvement of SKs and sphingosine-1-phosphate (S1P) in cellular processes. Dihydroceramide desaturase (Des1), the last enzyme in the de novo synthesis of ceramide (Cer), regulates the balance between dihydroceramides (dhCers) and Cers. Both SKs and Des1 have interest as therapeutic targets. Here we show that SKI II is a noncompetitive inhibitor (Ki = 0.3 μM) of Des1 activity with effect also in intact cells without modifying Des1 protein levels. Molecular modeling studies support that the SKI II-induced decrease in Des1 activity could result from inhibition of NADH-cytochrome b5 reductase. SKI II, but not the SK1-specific inhibitor PF-543, provoked a remarkable accumulation of dhCers and their metabolites, while both SKI II and PF-543 reduced S1P to almost undetectable levels. SKI II, but not PF543, reduced cell proliferation with accumulation of cells in the G0/G1 phase. SKI II, but not PF543, induced autophagy. These overall findings should be taken into account when using SKI II as a pharmacological tool, as some of the effects attributed to decreased S1P may actually be caused by augmented dhCers and/or their metabolites.

Metabolomics implicates altered sphingolipids in chronic pain of neuropathic origin

Neuropathic pain is a debilitating condition for which the development of effective treatments has been limited by an incomplete understanding of its chemical basis. We show by using untargeted metabolomics that sphingomyelin-ceramide metabolism is altered in the dorsal horn of rats with neuropathic pain and that the upregulated, endogenous metabolite N,N-dimethylsphingosine induces mechanical hypersensitivity in vivo. These results demonstrate the utility of metabolomics to implicate unexplored biochemical pathways in disease.

Apoptotic cell-derived factors induce arginase II expression in murine macrophages by activating ERK5/CREB

Apoptotic cell (AC)-derived factors alter the physiology of macrophages (MΦs) towards a regulatory phenotype, characterized by reduced nitric oxide (NO) production. Impaired NO formation in response to AC-conditioned medium (CM) was facilitated by arginase II (ARG II) expression, which competes with inducible NO synthase for L-arginine. Here we explored signaling pathways allowing CM to upregulate ARG II in RAW264.7 MΦs. Sphingosine-1-phosphate (S1P) was required and acted synergistically with a so far unidentified factor to elicit high ARG II expression. S1P activated S1P(2), since S1P(2) knockdown prevented ARG II upregulation. Furthermore, ERK5 knockdown attenuated CM-mediated ARG II protein induction. CREB was implicated as shown by EMSA analysis and decoy-oligonucleotides scavenging CREB in RAW264.7 MΦs, which blocked ARG II expression. We conclude that AC-derived S1P binds to S1P(2) and acts synergistically with other factors to activate ERK5 and concomitantly CREB. This signaling cascade shapes an anti-inflammatory MΦ phenotype by ARG II induction.

Acid ceramidase 1 expression correlates with a better prognosis in ER-positive breast cancer

OBJECTIVES

Ceramide and sphingosine mediate response to cancer therapy, inhibit cell growth and induce apoptosis in vitro. Only a few clinical data about the impact of ceramide and sphingosine iny vivo are available. We investigated the relevance of ceramide- and sphingosine-generating enzymes in breast cancer (acid ceramidase 1 (ASAH1), ceramide synthases 4 (LASS4) and 6 (LASS6)) by means of gene expression analysis.

METHODS

We analyzed differences in ASAH1, LASS4 and LASS6 on mRNA level between breast cancer subgroups using microarray data from 1581 tumor samples.

RESULTS

High ASAH1, LASS4 and LASS6 expression correlates with pathohistological grading (p < 0.001) and estrogen receptor (ER) status (p < 0.001). High ASAH1 expression was associated with a larger tumor size >2 cm (p = 0.003), while high LASS6 expression was correlated with ErbB2 negativity (p < 0.001). In survival analysis, we detected a significant better prognosis of patients with higher ASAH1 expression (p = 0.002) in the ER-positive subgroup. In contrast, expression of LASS4 or LASS6 did not show any prognostic impact. In the multivariate analysis, only ASAH1 expression (p = 0.002), tumor size (p < 0.0001) and ErbB2 positivity (p = 0.041) remained significant.

CONCLUSION

ASAH1 is an estrogen-dependent member of the sphingolipid metabolism, which might provide further prognostic information in ER-positive breast cancers.

Downregulating sphingosine kinase-1 for cancer therapy

BACKGROUND

The sphingolipids ceramide and sphingosine 1-phosphate (S1P) are key regulators of cell death and proliferation. The subtle balance between their intracellular levels is governed mainly by sphingosine kinase-1, which produces the pro-survival S1P. Sphingosine kinase-1 is an oncogene; is overexpressed in many tumors; protects cancer cells from apoptosis in vitro and in vivo; and its activity is decreased by anticancer therapies. Hence, sphingosine kinase-1 appears to be a target of interest for therapeutic manipulation.

OBJECTIVE

This review considers recent developments regarding the involvement of sphingosine kinase-1 as a therapeutic target for cancer, and describes the pharmacological tools currently available.

RESULTS/CONCLUSION

The studies described provide strong evidence that strategies to kill cancer cells via sphingosine kinase-1 inhibition are valid and could have a favorable therapeutic index.

Differential signaling of sphingosine derivatives in U937 human monocytes depends on the degree of N-methylation

Previously, we studied N,N-dimethyl-D-erythro-sphingosine (DMS)-induced cell death and signaling in U937 human monocytes; we found that DMS-induced sphingosine kinase- and PKC-independent apoptosis. In the present study, we studied apoptotic responses by three N-methyl derivatives of sphingosine: N-monomethyl-D-erythro-sphingosine (MMS), N,N,N-trimethyl-D-erythro-sphingosine (TMS), and D-erythro-sphingosine (SPH). The potency order in the apoptotic response was DMS>or=MMS>TMS>SPH. We compared cellular responses to the derivatives in terms of activities of MAPK signaling molecules, mitochondrial membrane potential (DeltaPsi(m)), and reactive oxygen species (ROS) generation. Our results suggest that the degree of N-methylation affects the apoptosis-inducing capacity and other related responses including MAPK modulation, DeltaPsi(m), and ROS generation. Dimethylation and monomethylation on the C2 amine of sphingosine enhance the apoptotic response; however, trimethylation induces differential modulation of signaling molecules and less cytotoxicity. Our investigation will be useful for understanding the actions of sphingolipids in apoptosis and for developing chemotherapeutics based on DMS structure.

N, N-dimethyl-D-erythro-sphingosine increases intracellular Ca2+ concentration via Na+-Ca2+-exchanger in HCT116 human colon cancer cells

N,N-dimethyl-D-erythro-sphingosine (DMS), an N-methyl derivative of sphingosine, is an inhibitor of protein kinase C (PKC) and sphingosine kinase (SK). In previous reports, DMS-induced intracellular Ca2+ increase concentration ([Ca2+]i) was studied in T lymphocytes, monocytes, astrocytes and neuronal cells. In the present study, we studied DMS-induced increase of [Ca2+]i in HCT116 human colon cancer cells. We found that the DMS-induced increase of [Ca2+]i in colon cancer cells is composed of Ca2+ release from intracellular Ca2+ stores and subsequent Ca2+ influx. The Ca2+ release is not related to modulation of inositol 1,4,5-trisphosphate (IP3) receptor or ryanodine receptor. On the other hand, the Ca2+ influx is mediated largely through Ca2+ channels sensitive to verapamil, nifedipine, Ga3+, and La3+. Furthermore, we found that the response is inhibited by bepridil and Ni2+, specific inhibitors of Na+-Ca2+-exchanger, suggesting involvement of Na+-Ca2+ exchanger in the DMS-induced [Ca2+]i increase in colon cancer cells. This inhibition was also observed in U937 monocytes, but not in 1321N1 astrocytes.

Characterization of N,N,-dimethyl-D-erythro-sphingosine-induced apoptosis and signaling in U937 cells: independence of sphingosine kinase inhibition

In the present study, we studied N,N-dimethyl-D-erythro-sphingosine (DMS)-induced cell death and its signaling mechanism in U937 human monocytes. We found that DMS induced cell death in a concentration-dependent manner, while sphingosine 1-phosphate did not. DMS also induced DNA fragmentation, nuclear disruption, and cytochrome c release from mitochondria in a concentration- and time-dependent manner, implying apoptotic cell death. DMS was found to increase mitochondrial membrane potential (MMP) immediately after addition of DMS and to decrease MMP at 2h after addition. However, sphingosine kinase inhibitors and PKC inhibitors did not induce cell death in U937 cells, a result that appears to exclude sphingosine kinase and PKC as target molecules of DMS in the cell death induction process. Furthermore, DMS modulated the activity of several signaling molecules. DMS induced activation of JNK and p38 MAP kinase, while it decreased the activity of ERK and Akt kinase. However, decrease of MMP, inhibition of JNK, p38 MAP kinase, ERK, or Akt with specific inhibitors could not mimic the DMS-induced cell death, implying multiple concerted processes are involved in DMS-induced cell death. In summary, DMS induced apoptotic cell death via modulation of MMP, JNK, p38 MAP kinase, ERK, and Akt kinase, but not through inhibition of sphingosine kinase or PKC in U937 cells.

Sphingolipid therapy in myocardial ischemia-reperfusion injury

Sphingolipids are known to play a significant physiological role in cell growth, cell differentiation, and critical signal transduction pathways. Recent studies have demonstrated a significant role of sphingolipids and their metabolites in the pathogenesis of myocardial ischemia-reperfusion injury. Our laboratory has investigated the cytoprotective effects of N,N,N-trimethylsphingosine chloride (TMS), a stable N-methylated synthetic sphingolipid analogue on myocardial and hepatic ischemia-reperfusion injury in clinically relevant in vivo murine models of ischemia-reperfusion injury. TMS administered intravenously at the onset of ischemia reduced myocardial infarct size in the wild-type and obese (ob/ob) mice. Following myocardial I/R, there was an improvement in cardiac function in the wild-type mice. Additionally, TMS also decreased serum liver enzymes following hepatic I/R in wild-type mice. The cytoprotective effects did not extend to the ob/ob mice following hepatic I/R or to the db/db mice following both myocardial and hepatic I/R. Our data suggest that although TMS is cytoprotective following I/R in normal animals, the cytoprotective actions of TMS are largely attenuated in obese and diabetic animals which may be due to altered signaling mechanisms in these animal models. Here we review the therapeutic role of TMS and other sphingolipids in the pathogenesis of myocardial ischemia-reperfusion injury and their possible mechanisms of cardioprotection.

Clinical relevance of ceramide metabolism in the pathogenesis of human head and neck squamous cell carcinoma (HNSCC): attenuation of C(18)-ceramide in HNSCC tumors correlates with lymphovascular invasion and nodal metastasis

It has been documented previously that defects in the generation of C(18)-ceramide, a product of ceramide synthase 1 (CerS1), also known as longevity assurance gene 1 (hLASS1), play important roles in the pathogenesis and/or progression of HNSCC. However, whether altered levels of ceramide generation in HNSCC tumors have any clinical relevance remains unknown. In this study, the levels of endogenous ceramides were measured in tumor tissues of 45 HNSCC patients as compared to their normal tissues using high-pressure liquid chromatography/mass spectrometry (LC/MS), and then possible link between ceramide levels and the clinical parameters of HNSCC were examined. The data showed that the levels of C(16)-, C(24)-, C(24:1)-ceramides were significantly elevated in the majority of tumor tissues compared to their normal tissues, while the levels of only C(18)-ceramide were significantly decreased in HNSCC tumors, especially in tumor tissues of male patients. Importantly, it was also shown here that decreased C(18)-ceramide levels in HNSCC tumor tissues were significantly associated with the higher incidences of lymphovascular invasion, and pathologic nodal metastasis. Importantly, attenuation of C(18)-ceramide was also positively linked to the higher overall stages of the primary HNSCC tumors. Therefore, these data suggest, for the first time, that the defects in the generation/accumulation of C(18)-ceramide might have important clinical roles in HNSCC, especially in lymphovascular invasion and nodal disease.

Sphingosine kinase-1--a potential therapeutic target in cancer

Sphingolipid metabolites play critical functions in the regulation of a number of fundamental biological processes including cancer. Whereas ceramide and sphingosine mediate and trigger apoptosis or cell growth arrest, sphingosine 1-phosphate promotes proliferation and cell survival. The delicate equilibrium between the intracellular levels of each of these sphingolipids is controlled by the enzymes that either produce or degrade these metabolites. Sphingosine kinase-1 is a crucial regulator of this two-pan balance, because it produces the prosurvival sphingosine 1-phosphate, and reduces the content of both ceramide and sphingosine, the proapoptotic sphingolipids. Sphingosine kinase-1 controls the levels of sphingolipids having opposite effects on cell survival/death, its gene was found to be of oncogenic nature, its mRNA is overexpressed in many solid tumors, its overexpression protects cells from apoptosis and its activity is decreased during anticancer treatments. Therefore, sphingosine kinase-1 appears to be a target of interest for therapeutic manipulation via its pharmacological inhibition. Strategies to kill tumor cells by increasing their ceramide and/or sphingosine content while blocking sphingosine 1-phosphate generation should have a favorable therapeutic index.

Dimethylsphingosine Regulates Intracellular pH and Ca2+ in Human Monocytes

Dimethylsphingosine (DMS) was first reported as an inhibitor of protein kinase C and later has been used as a specific inhibitor of sphingosine kinase. Furthermore, its anti-cancer effect has become a basis for development of chemotherapy. Nevertheless, its anti-neoplastic mechanism has poorly been understood. In the present study, we observed that DMS increased intracellular pH and Ca(2+) concentration in U937 human monocytes. To further characterize these DMS-induced actions, we employed structurally-related sphingolipids and specific pharmacological tools such as inhibitors of protein kinase C and Na(+)/H(+) exchanger and found that the two responses of DMS were mimicked by four stereoisomers of sphingosine and two isomers to dihydrosphingosine, but not with sphingosine 1-phosphate, sphingosyl-phosphorylcholine, and C2-ceramide. Furthermore, DMS-induced pH increase was independent of Na(+)/H(+) exchanger activity. We also characterized the interrelationship between DMS-induced pH increase and DMS-induced Ca(2+) increase. Since DMS is considered to be a good anti-cancer candidate, our characterization of DMS actions provides useful information for development of DMS chemotherapy.

Sphingosine activates protein kinase A type II by a novel cAMP-independent mechanism

Protein kinase A (PKA) has long been recognized as playing a major role in many regulatory processes in cells through its activation by the ubiquitous second messenger cAMP. We show here a novel mode of activation of PKA type II that is independent of cAMP and is, instead, dependent on sphingosine. PKA type II is specifically activated by sphingosine and its analog, dimethylsphingosine, but not by sphingosine-1-phosphate or other lipids. Like cAMP, sphingosine activates PKA holoenzyme but not the catalytic subunit alone, suggesting that the activation is mediated by the regulatory subunits. However, sphingosine-activated PKA, but not cAMP-activated PKA, is inhibited by phosphatidylserine, suggesting a distinct mechanism of activation. Furthermore, unlike cAMP, sphingosine does not induce the dissociation of PKA holoenzyme into catalytic and regulatory subunits. Modulation of sphingosine levels in vivo results in alteration in basal membrane-associated PKA activity consistent with a direct effect of membrane sphingosine on PKA type II. Importantly, sphingosine-dependent but not cAMP-dependent activation of PKA specifically phosphorylates Ser58 of the multifunctional adapter protein 14-3-3zeta, promoting the conversion of dimeric 14-3-3 to a monomeric state, thus potentially modulating several biological functions. These results define a new mode of PKA activation that is sphingosine-dependent and mechanistically different from the classical cAMP-dependent activation of PKA. Furthermore, they suggest that stimuli that induce sphingosine accumulation and modulate phospholipid content at the cell membrane have the potential to activate PKA, thereby inducing the phosphorylation of distinct substrates and biological activities.

The immune modulator FTY720 targets sphingosine–kinase‐dependent migration of human monocytes in response to amyloid beta‐protein and its precursor

Accumulation of inflammatory mononuclear phagocytes in Alzheimer's senile plaques, a hallmark of the innate immune response to beta-amyloid fibrils, can initiate and propagate neurodegeneration characteristic of Alzheimer's disease. Phagocytes migrate toward amyloid beta-protein involving formyl peptide receptor like-1-dependent signaling. Using human peripheral blood monocytes in Boyden chamber micropore filter assays, we show that the amyloid beta-protein- and amyloid beta-precursor protein-induced migration was abrogated by dimethylsphingosine, a sphingosine kinase inhibitor. Amyloid beta-protein stimulated in monocytes the gene expression for sphingosine-1-phosphate receptors 2 and 5, but not 1, 3, and 4. FTY720 that acts as a sphingosine-1-phosphate receptor agonist after endogenous phosphorylation by sphingosine kinase, as well as various neuropeptides that are known to be monocyte chemoattractants, dose-dependently inhibited amyloid beta-protein-induced migration. These data demonstrate that the migratory effects of beta-amyloid in human monocytes involve spingosine-1-phosphate signaling. Whereas endogenous neuropeptides may arrest and activate monocytes at sites of high beta-amyloid concentrations, interference with the amyloid beta-protein-dependent sphingosine-1-phosphate pathway in monocytes by FTY720, a novel immunomodulatory drug, suggests that FTY720 may be efficacious in beta-amyloid-related inflammatory diseases.

Internal residue loss produced by rearrangement of a novel cationic glycosphingolipid, glyceroplasmalopsychosine, in collision-induced dissociation

A novel plasmal conjugate of galactosylsphingosine (psychosine), Gro1(3)-O-plasmal-O-6Galbeta-sphingosine (glyceroplasmalopsychosine), was analyzed by electrospray ionization and liquid secondary ion mass spectrometry with low- or high-energy collision-induced dissociation (CID). In the product ion spectra of the [M + H](+) ions, [M + H - glycerol](+) ions arising from the loss of a glycerol were predominant. Unexpectedly, CID of the [M + H - glycerol](+) ion produced an outstanding ion, [(M + H - glycerol) - Hex](+), which required the loss of the galactose from inside the molecule. This ion was greatly reduced in the spectra of N,N-dimethyl derivatives, indicating that the [(M + H - glycerol) - Hex](+) ion is formed from an intramolecular rearrangement with migration of the plasmal residue to the free amino group of sphingosine. It would be expected that the rearrangement occurs simultaneously with the elimination of glycerol or a rearranged [M + H](+) ion leads to the elimination of glycerol, to form a Schiff base-type [M + H - glycerol](+) ion, from which the terminal galactose could be removed by the normal mechanism of glycosidic cleavage. On the other hand, the [M + Na - glycerol](+) ion derived from the sodiated molecule did not produce an ion corresponding to the rearrangement reaction, possibly owing to a higher stability of the sodiated ions against conformational changes.

Dimethylsphingosine increases cytosolic calcium and intracellular pH in human T lymphocytes

N,N-Dimethyl-D-erythro-sphingosine (DMS) is the N-methyl derivative of sphingosine; both are activators of sphingosine-dependent protein kinases. The aim of this work was to study the effect of DMS on cytosolic calcium and intracellular pH (pHi) in human T lymphocytes. The variations of calcium and pH were determined by fluorescence digital imaging using Fura-2-AM and BCECF-AM, respectively. DMS increased both pHi and Ca(2+)-cytoslic in human T lymphocytes. These effects were dose-dependent. This drug induced a fast increase in pHi and a release of calcium from different intracellular calcium pools than thapsigargin. DMS also induced a Ca(2+)-influx different from the store-operated calcium channels, since drug effect was not modified by 30 microM SKF 96365. The influx of calcium induced by DMS was completely blocked by preincubation in the presence of nickel, or lanthanum, while the increase in pHi was no affected. However, the presence of cadmium reduced but does not block Ca(2+)-influx. The inhibition of G-protein by 100 ng/mL pertussis toxin, and the inhibition of tyrosine kinases by genistein significantly reduced the cytosolic calcium increase induced by DMS by an inhibition of both, release of calcium from intracellular pools and influx from extracellular medium. The inhibition of pools emptiness by these drugs was related with the inhibition that they induce in the DMS cytosolic alcalinization. In summary, DMS increases pHi and as consequence releases calcium from intracellular pools, and it increases calcium-influx through a channel different from store-operated channel (SOC). Both cytosolic calcium and pHi increase are modulated by G-proteins and tyrosine kinases.

Sphingosine in apoptosis signaling

The sphingolipid metabolites ceramide, sphingosine, and sphingosine 1-phosphate contribute to controlling cell proliferation and apoptosis. Ceramide and its catabolite sphingosine act as negative regulators of cell proliferation and promote apoptosis. Conversely, sphingosine 1-phosphate, formed by phosphorylation of sphingosine by a sphingosine kinase, has been involved in stimulating cell growth and inhibiting apoptosis. As the phosphorylation of sphingosine diminishes apoptosis, while dephosphorylation of sphingosine 1-phosphate potentiates it, the role of sphingosine as a messenger of apoptosis is of importance. Herein, the effects of sphingosine on diverse signaling pathways implicated in the apoptotic process are reviewed.

Secretory Phospholipase A2 Inhibits Epidermal Growth Factor-Induced Receptor Activation

Secretory phospholipase A(2) (sPLA(2)) plays important roles in mediating various cellular processes, including cell proliferation, differentiation, apoptosis, and inflammatory response. In this study, we demonstrated that a basic sPLA(2) inhibits epidermal growth factor (EGF)-induced EGF receptor activation, as determined by autophosphorylation of EGF receptor, EGF-activated phospholipase D (PLD) activity, and phospholipase C-gamma(1) (PLC-gamma(1)) tyrosine phosphorylation in a human epidermoid carcinoma cell line, A-431. Treatment of cells with exogenous neutral sphingomyelinase (SMase) or a cell permeable ceramide analog, C(2)-ceramide, also caused similar inhibitory effects on EGF-induced activation of EGF receptor, tyrosine phosphorylation of PLC-gamma(1), and the activation of PLD. sPLA(2)-induced inhibition of EGF receptor was associated with arachidonic acid release, which was followed by an increase in intracellular ceramide formation. Both sPLA(2) and exogenous C(2)-ceramide are able to inhibit the proliferation of A-431. The data presented indicate for the first time that sPLA(2) downregulates the EGF receptor-mediated intracellular signal transduction that may be mediated by arachidonic acid and/or ceramide.

Prolactin induces calcium influx and release from intracellular pools in human T lymphocytes by activation of tyrosine kinases

The early events related to intracellular signals after prolactin (PRL) activation in T lymphocytes are not clearly established. The aim of this work was to study the effect of PRL in cytosolic calcium levels in human T lymphocytes. By using the dye FURA-2 AM, the variations in cytosolic Ca(2+) were studied in peripheral human T lymphocytes isolated from extracted blood from healthy donors. Fifty nanograms per milliliter PRL induces a small increase in cytosolic calcium. When the cells are preincubated overnight (16-20 h) in the presence of PRL, the increase in calcium is higher. This high increase is due to the release from intracellular pools and to the influx from the extracellular media. That is, after overnight incubation with PRL, calcium influx in T cells follows the capacitative model. Since PRL receptor (PRL-R) activation involves the tyrosine kinase pathway, we check calcium effect in the presence of genistein, a known inhibitor of tyrosine kinases. When cells are preincubated in the presence of 10 microM genistein, and PRL is immediately added, no increase in cytosolic calcium is observed. The presence of genistein also completely blocks the increase in cytosolic calcium stimulated by PRL after overnight incubation with PRL. In the presence of PRL and N,N-dimethyl-D-erythro-sphingosine (DMS), a stimulus that increases cytosolic calcium in T cells by tyrosine kinase stimulation, a high, even insignificant, calcium influx is induced. However, when the cells are incubated overnight in the presence of PRL, and then DMS is added, a significant increase in cytosolic calcium levels takes place. This increase is associated with an increase in calcium release from intracellular pools and an increase in calcium uptake. Genistein reduces the influx of external calcium induced by DMS after short incubation with PRL and significantly inhibits both, calcium pools empty, and calcium influx is induced by DMS after overnight incubation with PRL. In summary, PRL induces calcium influx in normal T lymphocytes. The influx is magnified after long PRL exposures, intracellular Ca(2+) pool-dependent, and activated through tyrosine kinases.

A novel plasmal conjugate to glycerol and psychosine ("glyceroplasmalopsychosine"): isolation and characterization from bovine brain white matter

A novel plasmal conjugate of glycosphingolipid having cationic lipid properties was isolated from the white matter of bovine brain. Linkage analysis of galactosyl residue by methylation, liquid secondary ion, and electrospray ionization mass spectrometry of intact and methylated derivatives, and by (1)H- and (13)C-NMR spectroscopy, identified the structure unambiguously as an O-acetal conjugate of plasmal to the primary hydroxyl group of glycerol and to the 6-hydroxyl group of galactosyl residue of beta-galactosyl 1-->1 sphingosine (psychosine). This novel compound is hereby termed "glyceroplasmalopsychosine"; its structure is shown below (see text).

The total syntheses of D-erythro-sphingosine, N-palmitoylsphingosine (ceramide), and glucosylceramide (cerebroside) via an azidosphingosine analog

The total synthesis of D-erythro-sphingosine (9) was performed by a chirospecific method starting from D-galactose via an azidosphingosine intermediate to give highly homogeneous (>99.9% C18:1) sphingosine base (9) which contained no observable olefin isomerization by product and was demonstrated to be optically pure by a novel method utilizing Mosher's acid. Ceramide (10) was prepared from this sphingosine (9) with highly homogeneous (99.8% C16:0) palmitic acid by two methods. The cerebroside glucosylceramide (23) was the next sphingolipid in this series to be synthesized in a highly homogeneous form. These three sphingolipids are currently being used for biophysical studies of the structures of their hydrated bio-molecular assemblies.

Differential regulation of protein tyrosine kinase on free radical production, granule enzyme release, and cytokine synthesis by activated murine peritoneal macrophages

The present study examined the regulatory effect of tyrosine kinase inhibitors (genistein, tyrphostin, and 2,5-dihydroxycinnamate) on the free radical production, granule enzyme release, and synthesis of interleukin (IL)-8 and granulocyte macrophage-colony stimulating factor (GM-CSF) in murine peritoneal macrophages exposed to different stimulators [10 ng/mL of IL-1, 1 microgram/mL of lipopolysaccharide (LPS), and 1 microM N-formyl-methionyl-leucyl-phenylalanine (fMLP)]. Protein tyrosine kinase (PTK) inhibitors attenuated the stimulated superoxide, hydrogen peroxide, and nitric oxide production in macrophages stimulated with IL-1, LPS, or fMLP. N,N-Dimethylsphingosine (DMS) alone stimulated superoxide and hydrogen peroxide production by intact macrophages, but at 45 microM the stimulatory effect on superoxide production was not found. In contrast, DMS attenuated nitric oxide production by macrophages. High concentrations of DMS, tyrphostin, and 2,5-dihydroxycinnamate showed cytotoxic effects. PTK inhibitors did not exhibit a significant effect on granule enzyme release induced by IL-1, whereas they attenuated the effect of LPS and fMLP on degranulation. Genistein and tyrphostin decreased the production of IL-8 and GM-CSF in macrophages activated by IL-1, whereas 2,5-dihydroxycinnamate did not affect it. The results suggest that tyrosine kinases exposed to IL-1, LPS, and fMLP may exert different modulatory actions on macrophage responses. The IL-1-activated macrophage responses, particularly degranulation, appear to be differently regulated by tyrosine kinases compared with the responses activated by LPS and fMLP.

Inhibition of Ca2+ release channel (ryanodine receptor) activity by sphingolipid bases: mechanism of action

Sphingosine inhibits the activity of the skeletal muscle Ca2+ release channel (ryanodine receptor) and is a noncompetitive inhibitor of [3H]ryanodine binding (Needleman et al., Am. J. Physiol. 272, C1465-1474, 1997). To determine the contribution of other sphingolipids to the regulation of ryanodine receptor activity, several sphingolipid bases were assessed for their ability to alter [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes and to modulate the activity of the Ca2+ release channel. Three lipids, N,N-dimethylsphingosine, dihydrosphingosine, and phytosphingosine, inhibited [3H]ryanodine binding to both skeletal and cardiac SR membranes. However, the potency of these three lipids and sphingosine was lower in rabbit cardiac membranes when compared to rabbit skeletal muscle membranes and when compared to sphingosine. Like sphingosine, the lipids inhibited [3H]ryanodine binding by greatly increasing the rate of dissociation of bound [3H]ryanodine from SR membranes, indicating that these three sphingolipid bases were noncompetitive inhibitors of [3H]ryanodine binding. These bases also decreased the activity of the Ca2+ release channel incorporated into planar lipid bilayers by stabilizing a long closed state. Sphingosine-1-PO4 and C6 to C18 ceramides of sphingosine had no significant effect on [3H]ryanodine binding to cardiac or skeletal muscle SR membranes. Saturation of the double bond at positions 4-5 decreased the ability of the sphingolipid bases to inhibit [3H]ryanodine binding 2-3 fold compared to sphingosine. In summary, our data indicate that other endogenous sphingolipid bases are capable of modulating the activity of the Ca2+ release channel and as a class possess a common mechanism of inhibition.

Depressant effects of ambroxol on lipopolysaccharide- or fMLP-stimulated free radical production and granule enzyme release by alveolar macrophages

In order to explore the depressant action of ambroxol, a bronchial expectorant, on the activated alveolar macrophage responses, its effect on lipopolysaccharide (LPS)- or N-formyl-methionyl-leucyl-phenylalanine (fMLP)- stimulated free radical production and granule enzyme release by rat lung alveolar macrophages was investigated. Ambroxol attenuated the 100 ng/ml LPS- or 1 microM fMLP-stimulated superoxide, H(2)O(2)and nitric oxide production and releases of acid phosphatase and lysozyme by alveolar macrophages. Ambroxol attenuated phorbol myristate acetate-stimulated superoxide and nitric oxide production that was inhibited by 100 nM staurosporine. N,N-dimethylsphingosine (DMS, 4.5 and 9 microM) alone stimulated superoxide production by macrophages, while 45 microM of the compound did not show a stimulatory effect. However, DMS decreased nitric oxide production in a dose-dependent manner. Ambroxol did not alter the DMS effect on free radical production that was affected by 10 microM genistein. A preincubation of macrophages with ambroxol (10 and 100 microM), staurosporine and genistein attenuated the elevation of [Ca(2+)](i)caused by LPS. The results suggest that ambroxol exerts a depressant effect on LPS- or fMLP-stimulated free radical production and granule enzyme release by rat alveolar macrophages, which may be attributed to its inhibitory action on the activation process, protein kinase C, but its action on protein tyrosine kinase is not suggested.

Induction and Suppression of Endothelial Cell Apoptosis by Sphingolipids: A Possible In Vitro Model for Cell-Cell Interactions Between Platelets and Endothelial Cells

Because sphingosine (Sph) is actively incorporated into platelets and rapidly converted to sphingosine 1-phosphate (Sph-1-P), which is then released extracellularly, it is important to study the effects of Sph and Sph-1-P on endothelial cells from the viewpoint of platelet-endothelial cell interaction. In this study, we found that Sph, as well as ceramide, induces apoptosis in human umbilical vein endothelial cells (HUVECs). In contrast, Sph-1-P acts as a HUVEC survival factor; this bioactive lipid was shown to protect HUVECs from apoptosis induced by the withdrawal of growth factors and to stimulate HUVEC DNA synthesis. In metabolic studies, [3H]Sph, incorporated into HUVECs, was converted to [3H]Cer and further to [3H]sphingomyelin in a time-dependent manner, whereas [3H]Sph-1-P formation from [3H]Sph was weak and transient. These findings in HUVECs are very different from those of platelets, which possess a highly active Sph kinase but lack Sph-1-P lyase. As a result, platelets abundantly store Sph-1-P, whereas HUVECs contain much less Sph-1-P. Finally, HUVECs, in contrast to platelets, failed to release Sph-1-P extracellularly, indicating that HUVECs themselves are not able to supply the survival factor Sph-1-P, but receive it from activated platelets. Our results suggest that platelets may maintain the integrity of endothelial cells by incorporating Sph and releasing Sph-1-P.

Induction and Suppression of Endothelial Cell Apoptosis by Sphingolipids: A Possible In Vitro Model for Cell-Cell Interactions Between Platelets and Endothelial Cells

Because sphingosine (Sph) is actively incorporated into platelets and rapidly converted to sphingosine 1-phosphate (Sph-1-P), which is then released extracellularly, it is important to study the effects of Sph and Sph-1-P on endothelial cells from the viewpoint of platelet-endothelial cell interaction. In this study, we found that Sph, as well as ceramide, induces apoptosis in human umbilical vein endothelial cells (HUVECs). In contrast, Sph-1-P acts as a HUVEC survival factor; this bioactive lipid was shown to protect HUVECs from apoptosis induced by the withdrawal of growth factors and to stimulate HUVEC DNA synthesis. In metabolic studies, [3H]Sph, incorporated into HUVECs, was converted to [3H]Cer and further to [3H]sphingomyelin in a time-dependent manner, whereas [3H]Sph-1-P formation from [3H]Sph was weak and transient. These findings in HUVECs are very different from those of platelets, which possess a highly active Sph kinase but lack Sph-1-P lyase. As a result, platelets abundantly store Sph-1-P, whereas HUVECs contain much less Sph-1-P. Finally, HUVECs, in contrast to platelets, failed to release Sph-1-P extracellularly, indicating that HUVECs themselves are not able to supply the survival factor Sph-1-P, but receive it from activated platelets. Our results suggest that platelets may maintain the integrity of endothelial cells by incorporating Sph and releasing Sph-1-P.

Endogenous substrates of sphingosine-dependent kinases (SDKs) are chaperone proteins: heat shock proteins, glucose-regulated proteins, protein disulfide isomerase, and calreticulin

Protein kinases whose activity is detectable only in the presence of sphingosine (Sph) or N,N'-dimethyl-Sph (DMS), but not in the presence of 15 other sphingolipids, phospholipids, and glycerolipids tested (Megidish, T., et al. (1995) Biochem. Biophys. Res. Commun. 216, 739-747), have been termed "sphingosine-dependent kinases" (SDKs). We showed previously that a purified SDK (termed "SDK1") phosphorylates a specific Ser position of adapter/chaperone protein 14-3-3 isoforms beta, eta, and zeta but not tau or sigma (Megidish, T., et al. (1998) J. Biol. Chem. 273, 21834-45). In this study we found the following: (i) other SDKs with different substrate specificities are present in cytosolic and membrane extracts of mouse Balb/c 3T3 (A31) fibroblasts. (ii) The activation of these SDKs is specific to D-erythro-Sph and its N-methyl derivatives, the effect of L-threo-Sph or its N-methyl derivatives is minimal, and nonspecific cationic amphiphiles have no effect at all. An SDK separated as fractions "TN31-33" phosphorylated a 50 kDa substrate which was identified as calreticulin, as well as two endogenous substrates with molecular mass 58 and 55 kDa, both identified as protein disulfide isomerase (PDI). This SDK, which specifically phosphorylates calreticulin and PDI, both molecular chaperones found at high levels in endoplasmic reticulum, is tentatively termed "SDK2". Another SDK activity was copurified with glucose-regulated protein (GRP) and heat shock proteins (HSP). One GRP substrate had the same amino acid sequence as GRP94 (synonym: endoplasmin); another HSP substrate had the same amino acid sequence as mouse HSP86 or HSP84, the analogues of human HSP90. An SDK activity separated and present in "fraction 42" from Q-Sepharose chromatography specifically phosphorylated GRP105 (or GRP94) and HSP68 but did not phosphorylate PDI or 14-3-3. This SDK is clearly different from other SDKs in its substrate specificity and is tentatively termed "SDK3". Interestingly, substrates of all these SDKs so far identified are molecular chaperones or adapters capable of binding to enzymes and key molecules involved in signal transduction, maintaining tertiary structure of bioactive molecules, or maintaining cellular homeostasis in response to environmental stress. Thus, the essential role of Sph and DMS is to activate molecular chaperones, thereby providing a link to the mechanism by which SDK activity regulates cellular homeostasis and signal transduction.

The formation of ceramide-1-phosphate during neutrophil phagocytosis and its role in liposome fusion

Ceramide, a product of agonist-stimulated sphingomyelinase activation, is known to be generated during the phagocytosis of antibody-coated erythrocytes by polymorphonuclear leukocytes. Agonist-stimulated formation of ceramide-1-phosphate is now shown to occur in 32PO4-labeled neutrophils. Ceramide-1-phosphate is formed by a calcium-dependent ceramide kinase, found predominately in the neutrophil plasma membrane. The neutrophil kinase is specific for ceramide because, in contrast to the bacterial diglyceride kinase, ceramide is not phosphorylated under conditions specific for diglyceride phosphorylation. Conversely, 1,2-diacylglycerol does not serve as substrate for the neutrophil ceramide kinase. Ceramide kinase activation occurs in a time-dependent fashion, reaching peak activity 10 min after formyl peptide stimulation and challenge with antibody-coated erythrocytes. The lipid kinase activity is optimal at pH 6.8. Because the formation of the phagolysosome is a critical event in phagocytosis, the effect of ceramide-1-phosphate in promoting the fusion of liposomes was determined. Both the addition of increasing concentrations of sphingomyelinase D and ceramide-1-phosphate promoted liposomal fusion. In summary, ceramide-1-phosphate is formed during phagocytosis through activation of ceramide kinase. Ceramide-1-phosphate may promote phagolysosome formation.

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.

Selectivity of sphingosine-induced apoptosis. Lack of activity of DL-erythyro-dihydrosphingosine

Sphingosine (Sph) is emerging as an intracellular regulator of cellular differentiation and apoptosis (Ohta, et al., Cancer Res., 55, 691-697, 1995). We have recently found that both Sph and its methylated derivative N,N-dimethylsphingosine (DMS) inhibit mitogen-activated protein kinase (MAPK) activity, suggesting that Sph-induced apoptosis may be mediated at least partly through inhibition of MAPK (Sakakura, et al., Int J Oncol, 11, 31-39, 1997). We report in this study that three stereoisomers, D-erythro-Sph, L-threo-Sph, and DL-erythro-dihydrosphingosine, were tested in induction of apoptosis and inhibition of MAPK activity in three different kinds of solid tumor cell lines. D-erythro-Sph was strongest in these effects among three compounds. L-threo-Sphingosine was partly active. On the other hand, DL-erythro-dihydrosphingosine was totally inactive. These results demonstrate the specificity of sphingosine action in induction of apoptosis and inhibition of MAPK, suggesting that Sph may play an important role as a physiological intracellular messenger of apoptosis in these cancer cells.

Chemoenzymic synthesis of potentially caged glycosphingolipids [correction of glycoshingolipids] (GSLs): potentially caged lyso-G(M3) and its analogue

In a previous work, a number of potentially caged sphingolipids and glycosphingolipids were chemically synthesized (Zehavi, 1997. Chem. Phys. Lipids 90, 55-61). The effects of GM3 and to a lesser extent, of lyso-GM3, are being studied. Considering that biologically inert, caged lyso-GM3 could be photolysed in the cell to release lyso-GM3, thus creating an attractive opportunity to study the subsequent sequence of events in the cell, the chemoenzymic synthesis of the potentially caged lyso-GM3, (5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylo nic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyloxy (1-1)- (2S,3R,4E)-2-(4-carboxymethyl-2-nitrobenzyloxycarbonyl-amino)-3-hy droxy-4- octadecene and of a potentially caged GM3 analogue, (5-acetamido-3,5-dideoxy-D- glycero-alpha-D-galacto-2-nonulopyranosylonic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyloxy -(1-3)- (2S, 3R, 4E)-2-(4-carboxymethyl-2-nitro-benzyloxycarbonylamino)-1- hydroxy-4-octadecene was undertaken. Both compounds, being 2-nitrobenzyloxycarbonyl derivatives, are light-sensitive and could be efficiently photolysed to the biologically active, corresponding lyso-GSLs.

Cholesterol is required for surface transport of influenza virus hemagglutinin

Transport from the TGN to the basolateral surface involves a rab/N-ethylmaleimide-sensitive fusion protein (NSF)/soluble NSF attachment protein (SNAP)/SNAP receptor (SNARE) mechanism. Apical transport instead is thought to be mediated by detergent-insoluble sphingolipid-cholesterol rafts. By reducing the cholesterol level of living cells by 60-70% with lovastatin and methyl-beta-cyclodextrin, we show that the TGN-to-surface transport of the apical marker protein influenza virus hemagglutinin was slowed down, whereas the transport of the basolateral marker vesicular stomatitis virus glycoprotein as well as the ER-to-Golgi transport of both membrane proteins was not affected. Reduction of transport of hemagglutinin was accompanied by increased solubility in the detergent Triton X-100 and by significant missorting of hemagglutinin to the basolateral membrane. In addition, depletion of cellular cholesterol by lovastatin and methyl-beta-cyclodextrin led to missorting of the apical secretory glycoprotein gp-80, suggesting that gp-80 uses a raft-dependent mechanism for apical sorting. Our data provide for the first time direct evidence for the functional significance of cholesterol in the sorting of apical membrane proteins as well as of apically secreted glycoproteins.

Modulation of skeletal muscle Ca2(+)-release channel activity by sphingosine

The effect of D-erythro-C18-sphingosine (sphingosine) and related compounds on the Ca(2+)-release channel (ryanodine binding protein) was examined on rabbit skeletal muscle membranes, on the purified ryanodine binding protein, and on the channel reconstituted into planar lipid bilayers. Sphingosine inhibited [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes in a dose-dependent manner similar to published results (R. A. Sabbadini, R. Betto, A. Teresi, G. Fachechi-Cassano, and G. Salviati. J. Biol. Chem. 267: 15475-15484, 1992). The sphingolipid also inhibited [3H]ryanodine binding to the purified ryanodine binding protein. Our results demonstrate that the inhibition of [3H]ryanodine binding by sphingosine is due to an increased rate of dissociation of bound [3H]ryanodine from SR membranes and a decreased rate of association of [3H]ryanodine to the high-affinity site. Unlike other modulators of the Ca(2+)-release channel, sphingosine can remove bound [3H]ryanodine from the high-affinity site within minutes. Sphingosine increased the rate of dissociation of [3H]ryanodine bound to a solubilized proteolytic fragment derived from the carboxy terminus of the ryanodine binding protein (cleavage at Arg4475). Sphingosine also inhibited the activity of the Ca(2+)-release channel incorporated into planar lipid bilayers. Taken together, the data provide evidence for a direct effect of sphingosine on the Ca(2+)-release channel. Sphingosine is a noncompetitive inhibitor at the high-affinity ryanodine binding site, and it interacts with a site between Arg4475 and the carboxy terminus of the Ca(2+)-release channel.

[14C]ceramide synthesis by sphingolipid ceramide N-deacylase: new assay for ceramidase activity detection

Sphingolipid ceramide N-deacylase (SCDase) is an enzyme which hydrolyzes the N-acyl linkage between fatty acid and sphingosine in ceramide of various glycosphingolipids and sphingomyelin. Recently the enzyme was found to catalyze the hydrolysis and its reverse reaction under different conditions. We report here an innovative method for synthesis of radioisotope-labeled ceramide with high specific activity using the reverse hydrolysis reaction of SCDase. More than 80% of free fatty acid was transferred to sphingosine when 1 nmol [14C]stearic acid and 2 nmol sphingosine were incubated with 5 microU SCDase at 37 degrees C for 20 h in 10 microliters of 25 mM phosphate buffer, pH 7.0, containing 0.3% Triton X-100. Free [14C]fatty acid and sphingosine were easily separated from synthesized [14C]ceramide by using a Sep-Pak Plus Silica and a Sep-Pak CM cartridge, respectively. We also developed a sensitive assay method for ceramidase using the [14C]ceramide prepared. The method consists of separation of the [14C]fatty acid released from [14C]ceramide by the action of enzyme on thin-layer chromatography followed by analysis and quantification with an imaging analyzer (BAS1000). This method was sensitive and qualitative and enabled the detection of ceramidase activity in invertebrates for the first time as well as in several human cancer cell lines.

Differential effects of synthetic sphingosine derivatives on melanoma cell motility, growth, adhesion and invasion in vitro

Cancer cell surface glycosphingolipids are considered to play a critical role in tumor growth and metastasis. However, the implications of glycoconjugates in the control of cell motility, which is considered to be involved in tumor invasion, are not fully understood. In this study, the effects of a series of synthetic sphingosine derivatives, obtained by the chemical transformation of azidosphingosines, on directional migration of K1735-M2 melanoma cells grown on type I collagen-coated surfaces were investigated. Following the application of 60 microM (2R, 3S, 4E)-2, 3-epimino-4-octadecen-3-ol (S4) the migration rate was 94 +/- 10 microns/day, compared with 377 +/- 22 microns/day in the control experiment. Six other analogues were not as potent. S4 also considerably down-modulated melanoma single cell motility. Inhibition of motile activity was associated with changes in the actin filament organization as well as with changes in the number and distribution of vinculin plaques. Moreover, the compound reduced the attachment abilities of melanoma cells to basement membrane Matrigel. Tumor cell invasion, however, was less affected and proliferation remained unimpaired after treatment with S4. These data suggest at least one intracellular mode of action of this particular synthetic sphingosine derivative by modulation of cytoskeletal organization. Melanoma cell motility and growth may be controlled independently via glycosphingolipids.

Sphingolipid-derived signalling modulators: interaction with phosphatidylserine

We previously described the synthesis of two deuterium-labelled sphingoid bases, which made it possible to perform NMR spectroscopy on this family of signalling modulators in membranes (Rigby, A.C, Barber, K.R and Grant, C.W.M. (1995) Biochim. Biophys. Acta 1240, 75-82). In the present work we sought to test the concept that such mediators may display altered physical behaviour through association with anionic lipids - as a possible mechanism of involvement in signal transduction. Lyso-dihydrogalactosylceramide with deuterium nuclei at C4 and C5 of the sphingosine backbone and at C'3 and C'4 of the galactose ring ([2H4]lyso-GalCer), and N,N-dimethylsphingosine with deuterated amino-methyl groups ([2H6]dimethylsphingosine), were assembled as minor components into unsonicated fluid bilayer membranes of 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol. The effect of (anionic) phosphatidylserine was considered in this zwitterionic host matrix. The results present a picture of rapidly reversible interaction. The (-) charged phosphatidylserine exerted readily-measurable control over the orientation of the carbohydrate residue of [2H4]lyso-GalCer. In contrast, surrounding (-) charges exerted little spectral influence at the level of C4 and C5 of the lyso-GalCer, membrane-inserted, backbone; or at the level of the amino group of dimethylsphingosine. It would appear that packing alterations induced by the phosphatidylserine/sphingoid base association can translate into sizeable spatial constraints in the neighbouring aqueous domain.