Metabolism of the unnatural anticancer lipid safingol, l-threo-dihydrosphingosine, in cultured cells

Abstract

We studied the metabolism of radioactively labeled safingol (l-threo-dihydrosphingosine) in primary cultured neurons, B104 neuroblastoma cells, and Swiss 3T3 fibroblasts, and compared it to that of its natural stereoisomer d-erythro-dihydrosphingosine. Both sphingoid bases are used as biosynthetic precursors for complex sphingolipids, albeit to different rates. Whereas a considerable amount of the natural sphingoid base is also directed to the catabolic pathway (20–66%, cell type dependent), only a minor amount of the nonnatural safingol is subjected to catabolic cleavage, most of it being N-acylated to the respective stereochemical variant of dihydroceramide. Interestingly, N-acylation of safingol to l-threo-dihydroceramide is less sensitive to fumonisin B1 than the formation of the natural d-erythro-dihydroceramide. In addition, safingol-derived l-threo-dihydroceramide, unlike its physiologic counterpart, is not desaturated. Most of it either accumulates in the cells (up to 50%) or is used as a biosynthetic precursor of the respective dihydrosphingomyelin (up to 45%). About 5% is, however, glucosylated and channeled into the glycosphingolipid biosynthetic pathway.Our results demonstrate that, despite its nonnatural stereochemistry, safingol is recognized and metabolized preferentially by enzymes of the sphingolipid biosynthetic pathway. Furthermore, our data suggest that the cytotoxic potential of safingol is reduced rather than enhanced via its metabolic conversion. We studied the metabolism of radioactively labeled safingol (l-threo-dihydrosphingosine) in primary cultured neurons, B104 neuroblastoma cells, and Swiss 3T3 fibroblasts, and compared it to that of its natural stereoisomer d-erythro-dihydrosphingosine. Both sphingoid bases are used as biosynthetic precursors for complex sphingolipids, albeit to different rates. Whereas a considerable amount of the natural sphingoid base is also directed to the catabolic pathway (20–66%, cell type dependent), only a minor amount of the nonnatural safingol is subjected to catabolic cleavage, most of it being N-acylated to the respective stereochemical variant of dihydroceramide. Interestingly, N-acylation of safingol to l-threo-dihydroceramide is less sensitive to fumonisin B1 than the formation of the natural d-erythro-dihydroceramide. In addition, safingol-derived l-threo-dihydroceramide, unlike its physiologic counterpart, is not desaturated. Most of it either accumulates in the cells (up to 50%) or is used as a biosynthetic precursor of the respective dihydrosphingomyelin (up to 45%). About 5% is, however, glucosylated and channeled into the glycosphingolipid biosynthetic pathway. Our results demonstrate that, despite its nonnatural stereochemistry, safingol is recognized and metabolized preferentially by enzymes of the sphingolipid biosynthetic pathway. Furthermore, our data suggest that the cytotoxic potential of safingol is reduced rather than enhanced via its metabolic conversion. Sphingolipids occur in all eukaryotic cells, where they are primarily components of the plasma membrane. Their ceramide backbone anchors them in the outer leaflet of the lipid bilayer. Their hydrophilic moiety, composed of carbohydrate chains or phosphorylcholine in the case of glycosphingolipids (GSLs) and sphingomyelin (SM), respectively, faces the extracellular space. Gangliosides are sialic acid-containing GSLs and are specifically abundant in the central nervous system, where they have been associated with development and maturation of the brain, neuritogenesis, synaptic transmission, memory formation, and synaptic aging (1Sonnino S. Chigorno V. Ganglioside molecular species containing C18- and C20-sphingosine in mammalian nervous tissues and neuronal cell cultures.Biochim. Biophys. Acta. 2000; 1469: 63-77Crossref PubMed Scopus (132) Google Scholar). Ceramide is not only a key intermediate in the synthetic and degradative pathway of sphingolipid metabolism but also a key player in the anti-proliferative cellular responses, including apoptosis, cell-cycle arrest, differentiation, and senescence (2Hannun Y.A. Functions of ceramide in coordinating cellular responses to stress.Science. 1996; 274: 1855-1859Crossref PubMed Scopus (1500) Google Scholar). In contrast, its catabolic intermediate sphingosine 1-phosphate (S1P) has been implicated as a second messenger in cellular proliferation and survival (3Olivera A. Spiegel S. Sphingosine-1-phosphate as second messenger in cell proliferation induced by PDGF and FCS mitogens.Nature. 1993; 365: 557-560Crossref PubMed Scopus (818) Google Scholar), and also in protection against ceramide-mediated apoptosis (4Cuvillier O. Pirianov G. Kleuser B. Vanek P.G. Coso O.A. Gutkind J.S. Spiegel S. Suppression of ceramide-mediated programmed cell death by sphingosine-1-phosphate.Nature. 1996; 381: 800-803Crossref PubMed Scopus (1357) Google Scholar). Thus, the dynamic balance between intracellular S1P and ceramide, also known as sphingolipid rheostat, appears to be essential for the determination of whether cells survive or die (5Spiegel S. Milstien S. Sphingosine 1-phosphate, a key cell signaling molecule.J. Biol. Chem. 2002; 277: 25851-25854Abstract Full Text Full Text PDF PubMed Scopus (512) Google Scholar). Safingol (l-threo-dihydrosphingosine) is a nonnatural isomer of dihydrosphingosine (sphinganine). Usually sphingoid bases contain two chiral centers, namely at carbon atoms 2 and 3. Natural sphingoid bases occur in the d-erythro (2S, 3R) configuration, but three additional nonnatural stereoisomers exist. Among the unnatural sphingoid bases, l-threo(2S, 3S)-dihydrosphingosine (safingol) is of particular interest due to its anticancer activity. It was shown to synergistically increase the toxicity of established chemotherapeutic agents in several cancer cells in vitro (6Schwartz G.K. Haimovitz-Friedman A. Dhupar S.K. Ehleiter D. Maslak P. Lai L. Loganzo F. Kelsen Jr., D.P. Fuks Z. Albino A.P. Potentiation of apoptosis by treatment with the protein kinase C-specific inhibitor safingol in mitomycin C-treated gastric cancer cells.J. Natl. Cancer Inst. 1995; 87: 1394-1399Crossref PubMed Scopus (109) Google Scholar), as well as in preclinical animal studies (7Kedderis L.B. Bozigian H.P. Kleeman J.M. Hall R.L. Palmer T.E. Harrison Jr., S.D. Susick Jr., R.L. Toxicity of the protein kinase C inhibitor safingol administered alone and in combination with chemotherapeutic agents.Fundam. Appl. Toxicol. 1995; 25: 201-217Crossref PubMed Scopus (0) Google Scholar) and in a phase I clinical trial (8Schwartz G.K. Ward D. Saltz L. Casper E.S. Spiess T. Mullen E. Woodworth J. Venuti R. Zervos P. Starniolo A.M. Kelsen D.P. A pilot clinical/pharmacological study of the protein kinase C-specific inhibitor safingol alone and in combination with doxorubicin.Clin. Cancer Res. 1997; 3: 537-543PubMed Google Scholar). The anticancer properties of safingol can be explained by its inhibitory effect on the activity of either protein kinase C (PKC) or sphingosine kinase. The competitive interaction of safingol with the regulatory phorbol binding domain of PKC could be correlated with partial inhibition of the multidrug resistance phenotype of certain tumor cells (9Sachs C.W. Safa A.R. Harrison S.D. Fine R.L. Partial inhibition of multidrug resistance by safingol is independent of modulation of P-glycoprotein substrate activities and correlated with inhibition of protein kinase C.J. Biol. Chem. 1995; 270: 26639-26648Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). As a competitive inhibitor of sphingosine kinase (10Buehrer B.M. Bell R.M. Sphingosine kinase: properties and cellular functions.Adv. Lipid Res. 1993; 26: 59-67PubMed Google Scholar), safingol could prevent the formation of S1P from sphingosine, which in turn is exclusively formed from ceramide (11Rother J. van Echten G. Schwarzmann G. Sandhoff K. Biosynthesis of sphingolipids: dihydroceramide and not sphinganine is desaturated by cultured cells.Biochem. Biophys. Res. Commun. 1992; 189: 14-20Crossref PubMed Scopus (130) Google Scholar). According to the sphingolipid rheostat concept, the imbalance in favor of ceramide at the expense of S1P should direct the cells into apoptosis. Considering the fact that safingol is a stereoisomer of an essential precursor of the sphingolipid biosynthetic pathway, the question concerning its metabolic fate arises. We have recently shown that in neuroblastoma cells (12Nätzker S. Heinemann T. Figueroa-Perez S. Schnieders B. Schmidt R.R. Sandhoff K. van Echten-Deckert G. cis-4-Methylsphingosine-phosphate induces apoptosis in neuroblastoma cells by opposite effects on p38 and ERK MAPKs.Biol. Chem. 2002; 383: 1885-1894Crossref PubMed Scopus (13) Google Scholar) as well as in primary cultured neurons (unpublished observations), the inhibitory effect of safingol (20 μM) on sphingosine kinase catalyzed phosphorylation of the synthetic sphingosine analog cis-4-methylsphingosine decreases over time. These findings indicate that, although safingol is a nonnatural stereoisomer, it appears to be efficiently metabolized by neuronal cells. Reports concerning the stereospecificity of enzymes involved in sphingolipid biosynthesis are, however, quite confusing. Stoffel and Bister (13Stoffel W. Bister K. Stereospecificities in the metabolic reactions of the four isomeric sphinganines (dihydrosphingosines) in rat liver.Hoppe-Seyler's Z. Physiol. Chem. 1973; 354: 169-181Crossref PubMed Scopus (45) Google Scholar) showed 30 years ago that in rat liver, l-threo-sphinganine is used like its natural analog as a precursor for the biosynthesis of SM and of cerebrosides. In contrast, Kok et al. (14Kok J.W. Nikolova-Karakashian N. Klappe K. Alexander K. Merrill Jr., A.H. Dihydroceramide biology. Structure-specific metabolism and intracellular localization.J. Biol. Chem. 1997; 272: 21128-21136Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar), who studied the metabolism of dihydroceramide in three different cell lines, reported that only the d-erythro-isomer is converted to more complex sphingolipids, the enzymes involved being highly stereo-selective. In a very recent study, Venkataraman and Futerman (15Venkataraman K. Futerman A.H. Comparison of the metabolism of l-erythro- and l-threo-sphinganines and ceramides in cultured cells and in subcellular fractions.Biochim. Biophys. Acta. Scholar) showed that, and are metabolized via the respective ceramides to only the natural d-erythro-isomer is converted to in rat as well as in cultured cells. As safingol has been used in phase clinical (8Schwartz G.K. Ward D. Saltz L. Casper E.S. Spiess T. Mullen E. Woodworth J. Venuti R. Zervos P. Starniolo A.M. Kelsen D.P. A pilot clinical/pharmacological study of the protein kinase C-specific inhibitor safingol alone and in combination with doxorubicin.Clin. Cancer Res. 1997; 3: 537-543PubMed Google Scholar), of its metabolism is radioactively labeled safingol and its metabolism in three different cell in with that of its natural stereochemical variant Our data demonstrate for the that like is used as a biosynthetic precursor for all complex by cultured cells. are essential between the metabolism of the natural and the nonnatural which in the to a that at the cells to the toxicity of the in the animal of the in was from and to Schwarzmann G. A and for of and Biophys. Acta. PubMed Scopus Google Scholar). was a from R. J. and E. of and and fumonisin B1 from and essential containing from was from and by The from and from was from of and from or cells cultured from of as Echten-Deckert G. A. A. Futerman A. Sandhoff K. a analog of sphinganine in a sphingolipid Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). cells by and by a in a The cells in containing and was to the to the of cells. or in cells used for metabolic The rat neuroblastoma B104 cell that in the central nervous D. Heinemann S. W. B. J. W. cell from the rat central nervous PubMed Scopus Google Scholar), as well as Swiss 3T3 cultured in with 2 and and cells in was was Swiss 3T3 and for in with or in with and 3T3 the cells cultured in was and the cells two with The cells labeled in containing and or B104 cells and Swiss 3T3 by of of either or cells three with and at for from cell with of for at by with for 2 at The lipid by on to and with the Sphingolipids by the and by In to lipid subjected to as Echten-Deckert G. and by 2000; PubMed Google Scholar). The lipid substrate was either the sphingolipid by as or the and to be The in a of and at the was by the of of of was and by phase The was to a that was with of SM was in The by as was used as The in a of and either from V. or from at the was by of The by on and by as a containing radioactively labeled from the and subjected to in of at The radioactively labeled sphingoid bases by as in on a 2 with a Lipid to be in into the by a of and a of at was with a of and in with was used as a and was at from to protein was as by A and sensitive for the of of protein the of PubMed Scopus Google Scholar) as a to lipid cell in of and used for protein Safingol is preferentially metabolized to and most of its is into and complex in cultured cells. the metabolism of have several metabolic studies in primary cultured The lipid from and as well as from studied in As in metabolic can be from sphinganine although the of certain Thus, the is used primarily as a biosynthetic precursor for that is metabolized to and to complex and In contrast, formed from safingol was channeled primarily into the formation of and to a to complex from safingol to be more than its in the cells Furthermore, of the from was in the from treatment that sphinganine stereoisomer is also directed the pathway G. D. V. Sandhoff K. In vitro and metabolism of Gangliosides and of Google L. R. G. of in cultured cells. The and sphingosine formed are for lipid 1993; PubMed Scopus Google Scholar). In contrast, the amount of safingol channeled to the catabolic pathway only less than 5% of the As shown in in Swiss 3T3 fibroblasts, like in primary cultured neurons, safingol is primarily N-acylated to that is used for the formation of a but amount is converted into A metabolism of safingol was in neuroblastoma B104 cells in tumor cells, unlike in the neurons or in the inhibitory Swiss 3T3 cells, a amount of safingol is channeled into the catabolic pathway, as by the amount of of the in the from the Thus, compared with its natural stereoisomer, the metabolism of safingol appears to be in all three cell for its into The is in the neuroblastoma cells than in primary cultured neurons and in Swiss 3T3 and respectively, of the in the from the of and from and d-erythro in primary cultured neurons, Swiss 3T3 fibroblasts, and B104 neuroblastoma and and B104 neuroblastoma not primary cultured Swiss 3T3 cultured in the of or cells and as in and lipid is as the of which in the case of to and of protein in and respectively, and in the case of to and of protein in and are from of three independent that that for the of only and in the lipid species that are in Both and in a B104 neuroblastoma not primary cultured Swiss 3T3 cultured in the of or cells and as in and lipid is as the of which in the case of to and of protein in and respectively, and in the case of to and of protein in and are from of three independent that that for the of only and in the lipid species that are in The of the of into as well as into the sphingolipid in primary cultured neurons is in The safingol-derived in the its and due to the of The amount of SM from safingol to increase due the of that was exclusively channeled into the pathway. 5% of the was from the and it was to increase with time. It is known that in the of an inhibitor of the bases sphinganine and sphingosine are directed the pathway Merrill Jr., A.H. metabolism and signaling Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). to can increase the amount of safingol that the catabolic pathway, the cells with of for to of the radioactively labeled As in the of a increase of from safingol was the amount of safingol was (up to over the than that of its natural d-erythro isomer Whereas the formation of and of SM and complex was by the to compared with to our was to and directed the formation of the respective to that a different namely an is also involved in the formation of unnatural It is well known that the sphingolipid biosynthetic pathway ceramide, for Jr., A.H. sphingolipid a but Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). The direct biosynthetic precursor of ceramide is the dihydroceramide. safingol is converted to a quite unnatural stereoisomer, it was to whether or not the desaturated or the In a the to from the and the and subjected to to the sphinganine and sphingosine As shown in only to of sphingosine and Considering the of 50%) in of the of the that of the dihydroceramide was In a second the sphingoid of all from safingol and As in the safingol-derived to exclusively contain the sphingoid base l-threo-sphinganine in results for neuroblastoma B104 cells and also Swiss 3T3 These findings demonstrate that the unnatural l-threo-dihydroceramide, in to its natural is not subjected to in cultured cells. Furthermore, as shown (14Kok J.W. Nikolova-Karakashian N. Klappe K. Alexander K. Merrill Jr., A.H. Dihydroceramide biology. Structure-specific metabolism and intracellular localization.J. Biol. Chem. 1997; 272: 21128-21136Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar), the results indicate that of the into the sphingoid backbone is not for metabolic of dihydroceramide to more complex In addition, that the lipid to safingol-derived and amount and These the of the respective containing In the study, that the anticancer lipid safingol is metabolized by cultured cells, despite its unnatural stereochemical We have shown that natural sphingoid bases to the are by the cells and are efficiently into cellular Echten-Deckert G. R. G. Schmidt R.R. Sandhoff K. of sphingolipid biosynthesis in primary cultured neurons by Biol. Chem. Full Text PDF PubMed Google Scholar). sphingoid bases can be and channeled to catabolic S. Merrill Jr., A.H. metabolism and cell J. 1996; PubMed Scopus Google Scholar). In all three cell like its natural was N-acylated to the respective was not in as as it has been in vitro and in cultured cells that dihydroceramide can l-threo-sphinganine (15Venkataraman K. Futerman A.H. Comparison of the metabolism of l-erythro- and l-threo-sphinganines and ceramides in cultured cells and in subcellular fractions.Biochim. Biophys. Acta. Scholar). the fate of the formed l-threo-dihydroceramide was a considerable amount was converted to In to findings in cells (15Venkataraman K. Futerman A.H. Comparison of the metabolism of l-erythro- and l-threo-sphinganines and ceramides in cultured cells and in subcellular fractions.Biochim. Biophys. Acta. Scholar), our results indicate that of the safingol-derived l-threo-dihydroceramide was desaturated. We have shown that the of the sphingoid base the activity in the of is than that of van Echten-Deckert G. J. Sandhoff K. E. Merrill Jr., A.H. of ceramide a dihydroceramide the of sphingosine at the of Biol. Chem. 1997; Full Text Full Text PDF Scopus Google Scholar). either the of dihydroceramide is or safingol-derived dihydroceramide the The fact is, the l-threo-dihydroceramide was not converted into its desaturated is an in as it the that the is converted into a The of safingol-derived l-threo-dihydroceramide to could cell it has been shown that ceramide stereoisomer is more in than its N. R. R. Z. of apoptosis in cells by stereoisomers and The ceramide is not for J. 1996; PubMed Scopus Google Scholar). In addition, the formation of the respective can be intermediate can be formed exclusively via the respective ceramides (11Rother J. van Echten G. Schwarzmann G. Sandhoff K. Biosynthesis of sphingolipids: dihydroceramide and not sphinganine is desaturated by cultured cells.Biochem. Biophys. Res. Commun. 1992; 189: 14-20Crossref PubMed Scopus (130) Google van Echten-Deckert G. J. Sandhoff K. E. Merrill Jr., A.H. of ceramide a dihydroceramide the of sphingosine at the of Biol. Chem. 1997; Full Text Full Text PDF Scopus Google Scholar). Interestingly, N-acylation of in to that of its natural counterpart, was to be at in that either the activity of S. P. A. Y.A. of the activity of rat Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) or the recently of and ceramide K. J. Merrill Jr., A.H. Futerman A.H. of and a mammalian of the in a fumonisin in mammalian cells.J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar) could be for the of safingol into In as as the was shown to activity the unnatural stereoisomers of sphingosine, it appears more that the N-acylation of the was reported to preferentially the of safingol-derived dihydroceramide and dihydrosphingomyelin in the In addition, is at in J. of the in and Res. Scopus Google Scholar). are to The natural in cells was shown to be preferentially channeled into the pathway of but not K. J. Merrill Jr., A.H. Futerman A.H. of and a mammalian of the in a fumonisin in mammalian cells.J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). In contrast, safingol-derived unlike its d-erythro stereoisomer, accumulates in the cells, that it is either quite or in the as (14Kok J.W. Nikolova-Karakashian N. Klappe K. Alexander K. Merrill Jr., A.H. Dihydroceramide biology. Structure-specific metabolism and intracellular localization.J. Biol. Chem. 1997; 272: 21128-21136Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar). The rather in as a considerable amount of safingol-derived dihydroceramide is converted into the respective and a but is channeled into the biosynthetic pathway The second is of of unnatural dihydroceramide isomer has not been reported (15Venkataraman K. Futerman A.H. Comparison of the metabolism of l-erythro- and l-threo-sphinganines and ceramides in cultured cells and in subcellular fractions.Biochim. Biophys. Acta. P. and of from rat Biol. Chem. 1995; Full Text Full Text PDF Scopus Google Scholar). The metabolic of safingol-derived l-threo-dihydroceramide is with preferentially the four stereoisomers of ceramide, rat was shown to only the natural d-erythro-isomer as substrate S. J. P. A. Y.A. of rat Lipid Res. 2002; Full Text Full Text PDF PubMed Google Scholar). of the the of the its substrate by S. J. P. A. Y.A. of rat Lipid Res. 2002; Full Text Full Text PDF PubMed Google Scholar). Thus, of safingol to l-threo-dihydroceramide appears to be a metabolic fate of safingol is its direct phosphorylation and and two namely a kinase and a are for degradative pathway. The amount of catabolic should be by the in the Our results suggest that, for neuroblastoma cells, only a minor of safingol is directed catabolic pathway. is not activity was to be in all tissues (13Stoffel W. Bister K. Stereospecificities in the metabolic reactions of the four isomeric sphinganines (dihydrosphingosines) in rat liver.Hoppe-Seyler's Z. Physiol. Chem. 1973; 354: 169-181Crossref PubMed Scopus (45) Google Lipid Res. 1993; 26: Google Scholar). In the brain, sphingosine kinase was also reported to specifically on the d-erythro-isomer (10Buehrer B.M. Bell R.M. Sphingosine kinase: properties and cellular functions.Adv. Lipid Res. 1993; 26: 59-67PubMed Google Scholar), in Swiss 3T3 fibroblasts, to be J.M. B.M. Bell R.M. of ceramide in induced by sphingoid Biol. Chem. Full Text PDF PubMed Google Scholar). the formed sphingoid not of the effects for S1P A. Schmidt R.R. Spiegel S. of intracellular and cellular Biol. Chem. Full Text PDF Google Scholar). The amount of safingol-derived in the of neuroblastoma cells that in cells, the kinase and the are not safingol our studies that safingol is metabolized in cultured cells being used as a biosynthetic precursor for all sphingolipid In to from studies (13Stoffel W. Bister K. Stereospecificities in the metabolic reactions of the four isomeric sphinganines (dihydrosphingosines) in rat liver.Hoppe-Seyler's Z. Physiol. Chem. 1973; 354: 169-181Crossref PubMed Scopus (45) Google K. Futerman A.H. Comparison of the metabolism of l-erythro- and l-threo-sphinganines and ceramides in cultured cells and in subcellular fractions.Biochim. Biophys. Acta. L. in tumor cell between and of ceramide Natl. Cancer Inst. 2000; PubMed Scopus Google Scholar), our data demonstrate that the formation of the intermediate can be in all three cell Interestingly, the N-acylation of safingol is different from the ceramide that its natural sphinganine stereoisomer to unlike reported in neuroblastoma cells L. in tumor cell between and of ceramide Natl. Cancer Inst. 2000; PubMed Scopus Google Scholar). an metabolic safingol was shown to cell and the was and L. in tumor cell between and of ceramide Natl. Cancer Inst. 2000; PubMed Scopus Google K. of protein kinase C and extracellular kinase in of 2002; PubMed Scopus Google Scholar). our data that safingol is only converted into like dihydroceramide and more complex sphingolipids, its activity most be explained by the formation of as L. in tumor cell between and of ceramide Natl. Cancer Inst. 2000; PubMed Scopus Google Scholar). our it appears more that the cytotoxic effect of safingol is a of sphingosine kinase on the (12Nätzker S. Heinemann T. Figueroa-Perez S. Schnieders B. Schmidt R.R. Sandhoff K. van Echten-Deckert G. cis-4-Methylsphingosine-phosphate induces apoptosis in neuroblastoma cells by opposite effects on p38 and ERK MAPKs.Biol. Chem. 2002; 383: 1885-1894Crossref PubMed Scopus (13) Google R. The potential of the 2002; PubMed Scopus Google Echten-Deckert G. A. Heinemann T. Schnieders B. cis-4-methylsphingosine the effect of in Swiss 3T3 Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) and PKC on the (9Sachs C.W. Safa A.R. Harrison S.D. Fine R.L. Partial inhibition of multidrug resistance by safingol is independent of modulation of P-glycoprotein substrate activities and correlated with inhibition of protein kinase C.J. Biol. Chem. 1995; 270: 26639-26648Abstract Full Text Full Text PDF PubMed Scopus (96) Google K. of protein kinase C and extracellular kinase in of 2002; PubMed Scopus Google Scholar). that the different cell used in different studies the different The for The also the of for from the of and K. Sandhoff for and study was by the fumonisin B1 glycosphingolipid sphingosine 1-phosphate sphingomyelin