Cell cycle arrest induced by an inhibitor of glucosylceramide synthase. Correlation with cyclin-dependent kinases

PDMP sensitizes neuroblastoma to paclitaxel by inducing aberrant cell cycle progression leading to hyperploidy

The sphingolipid ceramide has been recognized as an important mediator in the apoptotic machinery, and its efficient conversion to glucosylceramide has been associated with multidrug resistance. Therefore, inhibitors of glucosylceramide synthase are explored as tools for treatment of cancer. In this study, we used D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol to sensitize Neuro-2a murine neuroblastoma cells to the microtubule-stabilizing agent paclitaxel. This treatment resulted in a synergistic inhibition of viable cell number increase, which was based on a novel mechanism: (a) After a transient mitotic arrest, cells proceeded through an aberrant cell cycle resulting in hyperploidy. Apoptosis also occurred but to a very limited extent. (b) Hyperploidy was not abrogated by blocking de novo sphingolipid biosynthesis using ISP-1, ruling out involvement of ceramide as a mediator. (c) Cyclin-dependent kinase 1 and 2 activities were synergistically decreased on treatment. In conclusion, instead of inducing apoptosis through ceramide accumulation, D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol by itself affects cell cycle-related proteins in paclitaxel-arrested Neuro-2a cells resulting in aberrant cell cycle progression leading to hyperploidy.

Inhibition of sphingolipid synthesis impairs cellular activation, cytokine production and proliferation in human lymphocytes

The localisation of the T cell receptor and other signalling molecules in membrane microdomains (MM) is essential for the activation of T lymphocytes. These MM are stabilized by sphingolipids and cholesterol. It was recently shown that the activation of T lymphocytes leads to the confluence of small MM and the formation of an immunological synapse which is thought to be essential for a persistent activation and proliferation. We studied the effects of an inhibition of sphingolipid synthesis on T lymphocyte function. Both sphingolipid inhibitors, PDMP and myriocin, inhibited glucosphingolipids in whole cell lipid extracts and in MM. Both compounds inhibited the proliferation of superantigen-stimulated PBMC without inducing cell death. However, only the ceramide-like compound PDMP inhibited the expression of activation markers and the secretion of IFN-gamma which was not seen with myriocin treatment. The MM localisation of Lck and LAT was not significantly reduced in PDMP-treated cells. In conclusion, our results show that glucosphingolipids are necessary for cell growth of human T lymphocytes. However, inhibition of glucosphingolipid synthesis itself did not inhibit cellular activation. Our data show that glucosphingolipids - in contrast to cholesterol - are not essential for the stabilisation of MM.

Cholera toxin B-subunit prevents activation and proliferation of human CD4+ T cells by activation of a neutral sphingomyelinase in lipid rafts

The inhibition of human CD4+ T lymphocyte activation and proliferation by cholera toxin B-subunit (CTB) is a well-established phenomenon; nevertheless, the exact mechanism remained unclear. In the present study, we propose an explanation for the rCTB-induced inhibition of CD4+ T lymphocytes. rCTB specifically binds to GM1, a raft marker, and strongly modifies the lipid composition of rafts. First, rCTB inhibits sphingomyelin synthesis; second, it enhances phosphatidylcholine synthesis; and third, it activates a raft-resident neutral sphingomyelinase resembling to neutral sphingomyelinase type 1, thus generating a transient ceramide production. We demonstrated that these ceramides inhibit protein kinase Calpha phosphorylation and its translocation into the modified lipid rafts. Furthermore, we show that rCTB-induced ceramide production activate NF-kappaB. Combined all together: raft modification in terms of lipids, ceramide production, protein kinase Calpha inhibition, and NF-kappaB activation lead to CD4+ T cell inhibition.

A specific microdomain ("glycosynapse 3") controls phenotypic conversion and reversion of bladder cancer cells through GM3-mediated interaction of alpha3beta1 integrin with CD9

Cell motility is highly dependent on the organization and function of microdomains composed of integrin, proteolipid/tetraspanin CD9, and ganglioside (Ono, M., Handa, K., Sonnino, S., Withers, D. A., Nagai, H., and Hakomori, S. (2001) Biochemistry 40, 6414-6421; Kawakami, Y., Kawakami, K., Steelant, W. F. A., Ono, M., Baek, R. C., Handa, K., Withers, D. A., and Hakomori, S. (2002) J. Biol. Chem. 277, 34349-34358), later termed "glycosynapse 3" (Hakomori, S., and Handa, K. (2002) FEBS Lett. 531, 88-92, 2002). Human bladder cancer cell lines KK47 (noninvasive and nonmetastatic) and YTS1 (highly invasive and metastatic), both derived from transitional bladder epithelia, are very similar in terms of integrin composition and levels of tetraspanin CD9. Tetraspanin CD82 is absent in both. The major difference is in the level of ganglioside GM3, which is several times higher in KK47 than in YTS1. We now report that the GM3 level reflects glycosynapse function as follows: (i) a stronger interaction of integrin alpha3 with CD9 in KK47 than in YTS1; (ii) conversion of benign, low motility KK47 to invasive, high motility cells by depletion of GM3 by P4 (D-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol) treatment or by knockdown of CD9 by the RNA interference method; (iii) reversion of high motility YTS1 to low motility phenotype like that of KK47 by exogenous GM3 addition, whereby the alpha3-to-CD9 interaction was enhanced; (iv) low GM3 level activated c-Src in YTS1 or in P4-treated KK47, and high GM3 level by exogenous addition caused Csk translocation into glycosynapse, with subsequent inhibition of c-Src activation; (v) inhibition of c-Src by "PP2" in YTS1 greatly reduced cell motility. Thus, GM3 in glycosynapse 3 plays a dual role in defining glycosynapse 3 function. One is by modulating the interaction of alpha3 with CD9; the other is by activating or inhibiting the c-Src activity, possibly through Csk translocation. High GM3 level decreases tumor cell motility/invasiveness, whereas low GM3 level enhances tumor cell motility/invasiveness. Oncogenic transformation and its reversion can be explained through the difference in glycosynapse organization.

Glucosylceramide synthase blockade down-regulates P-glycoprotein and resensitizes multidrug-resistant breast cancer cells to anticancer drugs

Overexpression of glucosylceramide synthase (GCS), a pivotal enzyme in glycolipid biosynthesis, contributes to cancer cell resistance to chemotherapy. We previously showed that transfection of doxorubicin-resistant MCF-7-AdrR cells with GCS antisense restored cell sensitivity to doxorubicin and greatly enhanced sensitivity to vinblastine and paclitaxel. In that study, doxorubicin promoted generation of ceramide in MCF-7-AdrR/GCS antisense cells; the present study implicates factors in addition to ceramide that augment sensitivity to chemotherapy. Although GCS antisense cells showed enhanced ceramide formation compared with MCF-7-AdrR when challenged with paclitaxel, GCS antisense cells also showed a 10-fold increase in levels of intracellular drug (paclitaxel and vinblastine). In addition, transfected cells had dramatically decreased expression (80%) of P-glycoprotein and a 4-fold decrease in the level of cellular gangliosides. Chemical inhibition of GCS produced the same effects as antisense transfection: exposure of MCF-7-AdrR cells to the GCS inhibitor 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP, 5.0 micromol/L, 4 days) decreased ganglioside levels, restored sensitivity to vinblastine, enhanced vinblastine uptake 3-fold, and diminished expression of MDR1 by 58%, compared with untreated controls. A similar effect was shown in vinblastin-resistant KB-V0.01 cells; after 7 days with PPMP (10 micromol/L), MDR1 expression fell by 84% and P-glycoprotein protein levels decreased by 50%. MCF-7-AdrR cells treated with small interfering RNAs to specifically block GCS also showed a dramatic decrease in MDR1 expression. This work shows that limiting GCS activity down-regulates the expression of MDR1, a phenomenon that may drive the chemosensitization associated with blocking ceramide metabolism. The data suggest that lipids play a role in the expression of multidrug resistance.

Effect of ganglioside and tetraspanins in microdomains on interaction of integrins with fibroblast growth factor receptor

The functional interaction ("cross-talk") of integrins with growth factor receptors has become increasingly clear as a basic mechanism in cell biology, defining cell growth, adhesion, and motility. However, no studies have addressed the microdomains in which such interaction takes place nor the effect of gangliosides and tetraspanins (TSPs) on such interaction. Growth of human embryonal WI38 fibroblasts is highly dependent on fibroblast growth factor (FGF) and its receptor (FGFR), stably associated with ganglioside GM3 and TSPs CD9 and CD81 in the ganglioside-enriched microdomain. Adhesion and motility of these cells are mediated by laminin-5 ((LN5) and fibronectin (FN) through alpha3beta1 and alpha5beta1 integrin receptors, respectively. When WI38 cells or its transformant VA13 cells were adhered to LN5 or FN, alpha3beta1 or alpha5beta1 were stimulated, giving rise to signaling to activate FGFR through tyrosine phosphorylation and inducing cell proliferation under serum-free conditions without FGF addition. Types and intensity of signaling during the time course differed significantly depending on the type of integrin stimulated (alpha3beta1 versus alpha5beta1), and on cell type (WI38 versus VA13). Such effect of cross-talk between integrins and FGFR was influenced strongly by the change of GM3 and TSPs. (i) GM3 depletion by P4 caused enhanced tyrosine phosphorylation of FGFR and Akt followed by MAPK activation, without significant change of ceramide level. GM3 depletion also caused enhanced co-immunoprecipitation of FGFR with alpha3/alpha5/beta1 and of these integrins with CD9/CD81. (ii) LN5- or FN-dependent proliferation of both WI38 and VA13 was strongly enhanced by GM3 depletion and by CD9/CD81 knockdown by siRNA. Thus, integrin-FGFR cross-talk is strongly influenced by GM3 and/or TSPs within the ganglioside-enriched microdomain.

Interleukin-2-induced survival of natural killer (NK) cells involving phosphatidylinositol-3 kinase-dependent reduction of ceramide through acid sphingomyelinase, sphingomyelin synthase, and glucosylceramide synthase

Interleukin 2 (IL-2) rescued human natural killer (NK) KHYG-1 cells from apoptosis along with a reduction of ceramide. Conversely, an increase of ceramide inhibited IL-2-rescued survival. IL-2 deprivation-induced activation of acid sphingomyelinase (SMase) and inhibition of glucosylceramide synthase (GCS) and sphingomyelin synthase (SMS) were normalized by IL-2 supplementation. A phosphatidyl inositol-3 (PI-3) kinase inhibitor, LY294002, inhibited IL-2-rescued survival, but a mitogen-activated protein kinase inhibitor, PD98059, and an inhibitor of Janus tyrosine kinase/signal transducer and activator of transcription pathway, AG490, did not. LY294002 inhibited IL-2-induced reduction of ceramide through activation of acid SMase and inhibition of GCS and SMS, suggesting the positive involvement of PI-3 kinase in ceramide reduction through enzymatic regulation. Indeed, a constitutively active PI-3 kinase enhanced growth rate and ceramide reduction through inhibition of acid SMase and activation of GCS and SMS. Further, LY294002 inhibited IL-2-induced changes of transcriptional level as well as mRNA and protein levels in acid SMase and GCS but did not affect the stability of the mRNAs. These results suggest that PI-3 kinase-dependent reduction of ceramide through regulation of acid SMase, GCS, and SMS plays a role in IL-2-rescued survival of NK cells. (Blood. 2004;104:3285-3293)

Inhibition of glucosylceramide synthase does not reverse drug resistance in cancer cells

The multidrug-resistant cancer cell lines NCI/AdR(RES) and MES-SA/DX-5 have higher glycolipid levels and higher P-glycoprotein expression than the chemosensitive cell lines MCF7-wt and MES-SA. Inhibiting glycolipid biosynthesis by blocking glucosylceramide synthase has been proposed to reverse drug resistance in MDR cells by causing an increased accumulation of proapoptotic ceramide during treatment of cells with cytotoxic drugs. We treated both multidrug-resistant cell lines with the glucosylceramide synthase inhibitors PDMP (d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol), C9DGJ (N-nonyl-deoxygalactonojirimycin) or C4DGJ (N-butyl-deoxygalactonojirimycin). PDMP achieved a significant reversal of drug resistance in agreement with previous reports. However, the N-alkylated iminosugars C9DGJ and C4DGJ, which are more selective glucosylceramide synthase inhibitors than PDMP, failed to cause any reversal of drug resistance despite depleting glycolipids to the same extent as PDMP. Our results suggest that (a) inhibition of glucosylceramide synthase does not reverse multidrug resistance and (b) the chemosensitization achieved by PDMP cannot be caused by inhibition of glucosylceramide synthase alone.

Role for mammalian neutral sphingomyelinase 2 in confluence-induced growth arrest of MCF7 cells

Recently, we reported that neutral sphingomyelinase 2 (nSMase2) functions as a bona fide neutral sphingomyelinase and that overexpression of nSMase2 in MCF7 breast cancer cells caused a decrease in cell growth (Marchesini, N., Luberto, C., and Hannun, Y. A. (2003) J. Biol. Chem. 278, 13775-13783). In this study, the role of endogenous nSMase2 in regulating growth arrest was investigated. The results show that endogenous nSMase2 mRNA was up-regulated approximately 5-fold when MCF7 cells became growth-arrested at confluence, and total neutral SMase activity was increased by 119 +/- 41% with respect to control. Cell cycle analysis showed that up-regulation of endogenous nSMase2 correlated with G(0)/G(1) cell cycle arrest and an increase in total ceramide levels (2.4-fold). Analysis of ceramide species showed that confluence caused selective increases in very long chain ceramide C(24:1) (370 +/- 54%) and C(24:0) (266 +/- 81%) during arrest. The role of endogenous nSMase2 in growth regulation and ceramide metabolism was investigated using short interfering RNA (siRNA)-mediated loss-of-function analysis. Down-regulation of nSMase2 with specific siRNA increased the cell population of cells in S phase of the cell cycle by 59 +/- 14% and selectively reverted the effects of growth arrest on the increase in levels of very long chain ceramides. Mechanistically, confluence arrest also induced hypophosphorylation of the retinoblastoma protein (6-fold) and induction of p21(WAF1) (3-fold). Down-regulation of nSMase2 with siRNA largely prevented the dephosphorylation of the retinoblastoma protein and the induction of p21(WAF1), providing a link between the action of nSMase2 and key regulators of cell cycle progression. Moreover, studies on nSMase2 localization in MCF7 cells showed that nSMase2 distributed throughout the cells in subconfluent, proliferating cultures. In contrast, nSMase2 became nearly exclusively located at the plasma membrane in confluent, contact-inhibited cells. Hence, we demonstrate for the first time that nSMase2 functions as a growth suppressor in MCF7 cells, linking confluence to the G(0)/G(1) cell cycle check point.

TRAIL and Ceramide

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a clinically useful cytokine. TRAIL induces apoptosis in a wide variety of transformed cells, but does not cause toxicity to most normal cells. Recent studies show that death receptors (DR4 and DR5), decoy receptors (DcR1 and DcR2), and death inhibitors (FLIP, FAP-1, and IAP) are responsible for the differential sensitivity to TRAIL of normal and tumor cells. Several researchers have also shown that genotoxic agents, such as chemotherapeutic agents and ionizing radiation, enhance TRAIL-induced cytotoxicity by increasing DR5 gene expression or decreasing the intracellular level of FLIP, an antiapoptotic protein. Previous studies have shown that ceramide helps to regulate a cell's response to various forms of stress. Stress-induced alterations in the intracellular concentration of ceramide occur through the activation of a variety of enzymes that synthesize or catabolize ceramide. Increases in intracellular ceramide levels modulate apoptosis by acting through key proteases, phosphatases, and kinases. This review discusses the interaction between TRAIL and ceramide signaling pathways in regulating apoptotic death.

Activation of endothelial nitric-oxide synthase by tumor necrosis factor-alpha: a novel pathway involving sequential activation of neutral sphingomyelinase, phosphatidylinositol-3' kinase, and Akt

Activation of endothelial nitric-oxide synthase (eNOS) has been shown to occur through various pathways involving increases in the cytosolic Ca(2+) concentration, activation of the phosphatidylinositol-3' kinase/Akt pathway, as well as regulation by other kinases and by protein-protein interactions. We have recently reported that eNOS, expressed in an inducible HeLa Tet-off cell line, is activated by tumor necrosis factor-alpha (TNF-alpha) in a previously undescribed pathway that involves the lipid messenger ceramide. We have now characterized this pathway. We report here that eNOS activation in response to TNF-alpha correlated with phosphorylation of Akt at Ser 473 and of eNOS itself at Ser 1179. Akt and eNOS phosphorylation, as well as eNOS activation, were blocked by inhibitors of both phosphatidylinositol-3' kinase and neutral sphingomyelinase. In contrast, although acid sphingomyelinase was also stimulated by TNF-alpha, its inhibition was without effect. The activation of neutral sphingomyelinase triggered by TNF-alpha was insensitive to phosphatidylinositol-3' kinase inhibitors. Taken together, these results indicate that eNOS activation by TNF-alpha occurs through sequential activation of neutral sphingomyelinase and of the phosphatidylinositol-3' kinase/Akt pathway. The time course of eNOS activation induced through this pathway was markedly different from that triggered by ATP and epidermal growth factor, which activate eNOS through an increase in intracellular Ca(2+) concentration and through a sphingomyelinase-independent stimulation of the phosphatidylinositol-3' kinase/Akt pathway, respectively. The novel pathway of activation of eNOS described here may have broad biological relevance because neutral sphingomyelinase is activated not only by TNF-alpha but also by a variety of other physiological and pathological stimuli.

A role for ceramide, but not diacylglycerol, in the antagonism of insulin signal transduction by saturated fatty acids

Multiple studies suggest that lipid oversupply to skeletal muscle contributes to the development of insulin resistance, perhaps by promoting the accumulation of lipid metabolites capable of inhibiting signal transduction. Herein we demonstrate that exposing muscle cells to particular saturated free fatty acids (FFAs), but not mono-unsaturated FFAs, inhibits insulin stimulation of Akt/protein kinase B, a serine/threonine kinase that is a central mediator of insulin-stimulated anabolic metabolism. These saturated FFAs concomitantly induced the accumulation of ceramide and diacylglycerol, two products of fatty acyl-CoA that have been shown to accumulate in insulin-resistant tissues and to inhibit early steps in insulin signaling. Preventing de novo ceramide synthesis negated the antagonistic effect of saturated FFAs toward Akt/protein kinase B. Moreover, inducing ceramide buildup recapitulated and augmented the inhibitory effect of saturated FFAs. By contrast, diacylglycerol proved dispensable for these FFA effects. Collectively these results identify ceramide as a necessary and sufficient intermediate linking saturated fats to the inhibition of insulin signaling.

Cell cycle independent infection and gene transfer by recombinant Sendai viruses

A common problem for viral vectors in the field of somatic gene therapy is the dependence of an efficient cellular transduction on the cell cycle phase of target cells. An optimized viral vector system should therefore transduce cells in different cell cycle phases equally to improve transduction efficiencies. Recent observations that recombinant Sendai viruses (SeV) can infect a broad range of different tissues suggested SeV to be a good candidate for future gene therapeutic strategies in which dividing and non-dividing cells have to be reached. However, detailed data on the influence of distinct cell cycle phases on the infection of SeV or related viruses are missing. We report that synchronization of NIH 3T3 cells as well as contact inhibition of human fibroblast cells did not exhibit any negative influence on SeV infection rates. Furthermore, different attractive target tissues like human umbilical cord derived cells or primary human hepatocytes can be reached by SeV efficiently. As an important information for further cell cycle studies of paramyxoviruses we discovered surprisingly that the DNA polymerase inhibitor aphidicolin (induces a G(1)/M arrest) functions as an inhibitor of SeV but not of an adenoviral expression vector. In conclusion, the results demonstrate SeV based vector particles to be an ideal tool to reach equally cells coexisting in different cell cycle phases.

Increasing endogenous ceramide using inhibitors of sphingolipid metabolism maximizes ionizing radiation-induced mitochondrial injury and apoptotic cell killing

To enhance the killing effects of ionizing radiation, we amplified the endogenous ceramide signal in Jurkat cell cultures using 3 different inhibitors of sphingolipid metabolism: DL-PDMP, D-MAPP and imipramine. Of the various possible drug combinations, only DL-PDMP (20 microM) + imipramine (20 microM) and DL-PDMP (20 microM) + imipramine (20 microM) + D-MAPP (5 microM) induced a major increase in ceramide levels, reaching 240% and 340% of control values, respectively, after incubation for 48 hr. With these models, we demonstrate that endogenously formed ceramide triggers time- and concentration-dependent apoptosis through induction of mitochondrial injury and activation of the caspase pathway. Cellular dysfunction includes alterations to the cellular redox potential, as assessed by the generation of ROS and total glutathione depletion, and a drop in Delta Psi(m). A parallel elevation of mitochondrial ceramide levels was also observed. The combination of DL-PDMP + imipramine +/- D-MAPP with 10 Gy irradiation produced cumulative effects leading to apoptosis via mitochondrial collapse and activation of the caspase cascade. The association efficiency was confirmed in normal and acid sphingomyelinase-deficient lymphoid cell lines. Taken together, these results suggest that increasing endogenous ceramide levels may potentially be very valuable when combined with ionizing radiation in tumor therapy.

Decreased activity and enhanced nuclear export of CCAAT-enhancer-binding protein beta during inhibition of adipogenesis by ceramide

To identify novel molecular mechanisms by which ceramide regulates cell differentiation, we examined its effect on adipogenesis of 3T3-L1 preadipocytes. Hormonal stimulation of 3T3-L1 preadipocytes induced formation of triacylglycerol-laden adipocytes over 7 days; in part, via the co-ordinated action of CCAAT-enhancer-binding proteins alpha, beta and delta (C/EBP-alpha, -beta and -delta) and peroxisome-proliferator-activated receptor gamma (PPARgamma). The addition of exogenous N-acetylsphingosine (C2-ceramide) or increasing endogenous ceramide levels inhibited the expression of C/EBPalpha and PPARgamma, and blocked adipocyte development. C2-ceramide did not decrease the cellular expression of C/EBPbeta, which is required for expression of C/EBPalpha and PPARgamma, but significantly blocked its transcriptional activity from a promoter construct after 24 h. The ceramide-induced decrease in the transcriptional activity of C/EBPbeta correlated with a strong decrease in its phosphorylation, DNA-binding ability and nuclear localization at 24 h. However, ceramide did not change the nuclear level of C/EBPbeta after a period of 4 or 16 h, suggesting that it was not affecting nuclear import. CRM1 (more recently named 'exportin-1') is a nuclear membrane protein that regulates protein export from the nucleus by binding to a specific nuclear export sequence. Leptomycin B is an inhibitor of CRM1/exportin-1, and reversed the ceramide-induced decrease in nuclear C/EBPbeta at 24 h. Taken together, these data support the hypothesis that ceramide may inhibit adipogenesis, at least in part, by enhancing dephosphorylation and premature nuclear export of C/EBPbeta at a time when its maximal transcriptional activity is required to drive adipogenesis.

Gangliosides are functional nerve cell ligands for myelin-associated glycoprotein (MAG), an inhibitor of nerve regeneration

Myelin-associated glycoprotein (MAG) binds to the nerve cell surface and inhibits nerve regeneration. The nerve cell surface ligand(s) for MAG are not established, although sialic acid-bearing glycans have been implicated. We identify the nerve cell surface gangliosides GD1a and GT1b as specific functional ligands for MAG-mediated inhibition of neurite outgrowth from primary rat cerebellar granule neurons. MAG-mediated neurite outgrowth inhibition is attenuated by (i) neuraminidase treatment of the neurons; (ii) blocking neuronal ganglioside biosynthesis; (iii) genetically modifying the terminal structures of nerve cell surface gangliosides; and (iv) adding highly specific IgG-class antiganglioside mAbs. Furthermore, neurite outgrowth inhibition is mimicked by highly multivalent clustering of GD1a or GT1b by using precomplexed antiganglioside Abs. These data implicate the nerve cell surface gangliosides GD1a and GT1b as functional MAG ligands and suggest that the first step in MAG inhibition is multivalent ganglioside clustering.

Regulation of phospholipase C-gamma activity by glycosphingolipids

Glycosphingolipid-enriched domains are hot spots for cell signaling within plasma membranes and are characterized by the enrichment of glycosphingolipids. A role for glucosylceramide-based glycosphingolipids in phospholipase C-mediated inositol 1,4,5-trisphosphate formation has been previously documented. These earlier studies utilized a first generation glucosylceramide synthase inhibitor to deplete cells of their glycosphingolipids. Recently, more active and specific glucosylceramide synthase inhibitors, including d-threo-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidinopropanol (d-t-EtDO-P4), have been designed. d-t-EtDO-P4 has the advantage of blocking glucosylceramide synthase at low nanomolar concentrations but does not cause secondary elevations in cell ceramide levels. In the present study, d-t-EtDO-P4 depleted cellular glucosylceramide and lactosylceramide in cultured ECV304 cells at nanomolar concentrations without obvious cellular toxicity. The expression of several signaling proteins was evaluated in glycosphingolipid-depleted ECV304 cells to study the role of glycosphingolipids in phospholipase C-mediated signaling. No difference was observed in the cellular expression of phospholipase C-gamma between controls and glycolipid-depleted cells. Western blot analysis, however, revealed that depletion of endogenous glycosphingolipids in cultured ECV304 cells with d-t-EtDO-P4 induced tyrosine phosphorylation of phospholipase C-gamma in a concentration-dependent manner with maximum induction at 100 nm. The phosphorylation of phospholipase C-gamma induced by d-t-EtDO-P4 was abolished by exogenously added glucosylceramide, consistent with a specific glycosphingolipid-phospholipase C-gamma interaction. The phospholipase C-gamma phosphorylation was maximally enhanced by bradykinin when cells were exposed to 100 nm d-t-EtDO-P4. The measurement of cellular activity of phospholipase C-gamma, by myo-inositol 1,4,5-trisphosphate radioreceptor assay, demonstrated that depletion of glucosylceramide-based glycosphingolipids in cultured ECV304 cells with d-t-EtDO-P4 resulted in significantly increased formation of inositol 1,4,5-trisphosphate above base line, and an increased sensitivity of phospholipase C-gamma to bradykinin stimulation. Thus, the activation of phospholipase C-gamma is negatively regulated by membrane glycosphingolipids in ECV304 cells.

Biochemical mechanisms of the generation of endogenous long chain ceramide in response to exogenous short chain ceramide in the A549 human lung adenocarcinoma cell line. Role for endogenous ceramide in mediating the action of exogenous ceramide

Treatment of A549 cells with C(6)-ceramide resulted in a significant increase in the endogenous long chain ceramide levels, which was inhibited by fumonisin B1 (FB1), and not by myriocin (MYR). The biochemical mechanisms of generation of endogenous ceramide were investigated using A549 cells treated with selectively labeled C(6)-ceramides, [sphingosine-3-(3)H]d-erythro-, and N-[N-hexanoyl-1-(14)C]d-erythro-C(6)-ceramide. The results demonstrated that (3)H label was incorporated into newly synthesized long chain ceramides, which was inhibited by FB1 and not by MYR. Interestingly, the (14)C label was not incorporated into long chain ceramides. Taken together, these results show that generation of endogenous ceramide in response to C(6)-ceramide is due to recycling of the sphingosine backbone of C(6)-ceramide via deacylation/reacylation and not due to the elongation of its fatty acid moiety. Moreover, the generation of endogenous long chain ceramide in response to C(6)-ceramide was completely blocked by brefeldin A, which causes Golgi disassembly, suggesting a role for the Golgi in the metabolism of ceramide. In addition, the generation of endogenous ceramide in response to short chain exogenous ceramide was induced by d-erythro- but not l-erythro-C(6)-ceramide, demonstrating the stereospecificity of this process. Interestingly, several key downstream biological activities of ceramide, such as growth inhibition, cell cycle arrest, and modulation of telomerase activity were induced by d-erythro-C(6)-ceramide, and not l-erythro-C(6)-ceramide (and inhibited by FB1) in A549 cells, suggesting a role for endogenous long chain ceramide in the regulation of these responses.

Transfection of glucosylceramide synthase antisense inhibits mouse melanoma formation

MEB4 murine melanoma cells synthesize G(M3) as the major ganglioside. Inhibition of G(M3) synthesis by a specific glucosylceramide synthase inhibitor resulted in reduced tumorigenicity and metastatic potential of these cells. We used a molecular approach--antisense transfection targeting the glucosylceramide synthase gene--to regulate glycosphingolipid synthesis by MEB4 cells and examine the influence on tumor formation. Antisense transfection inhibited the synthesis of the direct product of glucosylceramide synthase, glucosylceramide, and consequently G(M3) ganglioside, by MEB4 cells, reducing the concentration of G(M3) in the transfectants by up to 58%. Although neither morphology nor proliferation kinetics of the cultured cells was affected, the inhibition of glycosphingolipid synthesis and reduction of total ganglioside content caused a striking reduction in melanoma formation in mice. Only 1/60 (2%) of mice injected ID with 10(4) antisense-transfected MA173 cells formed a tumor, compared to 31/60 (52%) of mice receiving MEB4 cells and 7/15 (47%) of mice receiving the MS2 sense-transfected cells (p < 0.001 and p = 0.005, respectively). These findings demonstrate that stable transfection of glucosylceramide synthase antisense reduces cellular glycosphingolipid levels and reduces tumorigenicity, providing further experimental support for an enhancing role of gangliosides in tumor formation.

Possible implication of Golgi-nucleating function for the centrosome

The Golgi apparatus breaks down at mitosis, resulting in the dispersal of Golgi-resident proteins. In NRK cells, however, subsets of both TGN38 and golgin-97, but not ManII and GM130, remained associated with the centrosome throughout the cell cycle. This centrosome association of TGN38 and golgin-97 was not disrupted by treatment with brefeldin A, additional inducers of retrograde trafficking and inhibitors of either kinases or protein phosphatases. Anchoring of the Golgi apparatus within the juxtanuclear region depends on microtubules; the association of TGN38 and golgin-97 subsets with the centrosome, however, was insensitive to nocodazole treatment. Drugs such as PDMP, which block Golgi dispersal both by nocodazole, despite microtubule depolymerization, and by inducers of retrograde trafficking, strengthened the microtubule-nucleating activity of the centrosome. These observations cumulatively suggest the centrosome is implicated in nucleation of the Golgi apparatus through interactions with Golgi-resident proteins, such as TGN38 and golgin-97.

Lactosylceramide is essential for the osteoclastogenesis mediated by macrophage-colony-stimulating factor and receptor activator of nuclear factor-kappa B ligand

Glycosphingolipids and their metabolites play important roles in a variety of biological processes. Several signal molecules are localized in a glycolipid-enriched microdomain on the cell surface, and their signals are regulated by the glycolipid composition. However, the function of glycolipids in osteoclastogenesis has not been clearly understood. We found that D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), a glucosylceramide synthase inhibitor, completely inhibits the osteoclast formation induced by macrophage-colony-stimulating factor and receptor activator of nuclear factor-kappa B ligand (RANKL) in a dose-dependent manner. Expression of RANK, the receptor of RANKL, induced by macrophage colony-stimulating factor, was reduced markedly in D-PDMP-treated cells. d-PDMP also inhibited the phosphorylation of the inhibitor of nuclear factor-kappa B and extracellular signal-regulated kinase 1/2 induced by RANKL. In several experiments with the addition of glycolipids to D-PDMP-treated purified bone marrow cells, lactosylceramide (LacCer) strongly affected the differentiation into tartrate-resistant acid phosphatase mononucleated cells, but not positive multinucleated cells. GM3 and GM1 also recovered, but less effectively compared with LacCer. Moreover, exogenous LacCer recovered the reduced expression of RANK and the phosphorylation of inhibitor of NF-kappa B and extracellular signal-regulated kinase 1/2 after stimulation by RANKL at the same level of cells without D-PDMP treatment. Our data suggest that glycosphingolipids, especially LacCer, are necessary for the initiation step of RANKL-induced osteoclastogenesis.

Updates on functions of ceramide in chemotherapy-induced cell death and in multidrug resistance

The sphingolipid ceramide, a bioeffector lipid, is known to regulate anti-proliferative responses, such as apoptosis, growth arrest, differentiation and senescence in various human cancer cell lines. Previous studies have demonstrated that many anti-cancer agents cause elevation of endogenous ceramide levels generated via the de novo pathway and/or the hydrolysis of sphingomyelin, accompanied by apoptotic cell death in human cancer cells. It has also been shown that decreased levels of endogenous ceramide by over-expression of glucosylceramide synthase, which clears ceramide levels by incorporating it into glucosylceramide, results in the development of a multidrug resistant phenotype in cancer cells. These studies demonstrate that ceramide plays important roles in the response of cancer cells to chemotherapeutic drugs. The goal of this review is to provide an update on recent studies which shed new light into the roles of ceramide in chemotherapy-induced apoptosis and in multidrug resistance (MDR) in human cancer cells.

Lactosylceramide mediates shear-induced endothelial superoxide production and intercellular adhesion molecule-1 expression

Laminar shear stress activates NADPH oxidase in vascular endothelial cells (ECs), and the generated superoxide radicals (O2(-.) are known to be involved in intercellular adhesion molecule (ICAM)-1 expression. In this study, the role of a glycosphingolipid (GSL), lactosylceramide (LacCer), as a second messenger in the shear-induced O2(-.) generation and ICAM-1 expression was examined. It is known that glucosylceramide synthase (GlcT-1) catalyzes the synthesis of glucosylceramide (GlcCer) from ceramide, and subsequently lactosylceramide synthase (GalT-2) synthesizes LacCer from GlcCer. We observed that exposing cultured human umbilical vein ECs (HUVECs) to fluid shear stress (20 dyn/cm(2) for 30 min) activated GalT-2. Shear stress also increased EC O2(-.) generation, that peaked at 30 min, and surface ICAM-1 protein expression at 6 h post-shear. EC preincubation with the antioxidant N-acetylcysteine (NAC; 20 mM for 2 h) completely abolished the shear-induced O2(-.) production and significantly inhibited ICAM-1 expression. EC preincubation with D-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of the GSL glycosyltransferases GlcT-1 and GalT-2, abrogated the shear-induced activation of GalT-2. D-PDMP also abolished the shear-induced O2(-.) production and ICAM-1 expression. We conclude that laminar shear stress activates GalT-2 to produce LacCer. In turn, LacCer activates NADPH oxidase, which produces O2(-.), and O2(-.) mediates the shear-induced increase in ICAM-1 expression. Thus, LacCer may play an important role in hemodynamic force-induced pathological conditions, such as atherosclerosis and ischemia/reperfusion injury.

Signaling pathways activated by daunorubicin

The anthracycline daunorubicin is widely used in the treatment of acute nonlymphocytic leukemia. The drug has, of course, been the object of intense basic research, as well as preclinical and clinical study. As reviewed in this article, evidence stemming from this research clearly demonstrates that cell response to daunorubicin is highly regulated by multiple signaling events, including a sphingomyelinase-initiated sphingomyelin-ceramide pathway, mitogen-activated kinase and stress-activated protein/c-Jun N-terminal kinase activation, transcription factors such as nuclear factor kappa B, as well as the Fas/Fas-ligand system. These pathways are themselves influenced by a number of lipid products (diacylglycerol, sphingosine-1 phosphate, and glucosyl ceramide), reactive oxygen species, oncogenes (such as the tumor suppressor gene p53), protein kinases (protein kinase C and phosphoinositide-3 kinase), and external stimuli (hematopoietic growth factors and the extracellular matrix). In light of the complexity and diversity of these observations, a comprehensive review has been attempted toward the understanding of their individual implication (and regulation) in daunorubicin-induced signaling. (Blood. 2001;98:913-924)

Glucosylceramide synthase inhibition enhances vincristine-induced cytotoxicity

As a strategy to enhance tumor cell sensitivity to vincristine, we tested whether modulation of sphingolipid metabolism would alter vincristine cytotoxicity since this is linked to accumulation of the intermediate metabolite, ceramide. We blocked ceramide metabolism in a series of variably vincristine-resistant cell lines derived from CCRF-CEM leukemia cells using an inhibitor of glucosylceramide synthase, DL-threo-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol (PPPP). PPPP alone (1.0 microM), while nearly completely blocking glucosylceramide synthesis, was not toxic and did not increase cellular ceramide levels. Vincristine alone was toxic, caused apoptosis or programmed cell death (PCD) and caused an elevation in ceramide levels. Strikingly, the combination of PPPP and vincristine resulted in a further increase, over that of vincristine alone, of (i) cellular ceramide concentration, (ii) cytotoxicity associated with PCD and (iii) G2/M cell-cycle arrest. PPPP had no effect on P-glycoprotein expression or function. We conclude that vincristine cytotoxicity occurs in part through a ceramide-dependent mechanism, resulting in both G2/M block as well as PCD, and that the blockade of glucosylceramide synthase, in itself not toxic, causes augmented accumulation of ceramide resulting from vincristine exposure, which in turn maximizes ceramide-dependent, vincristine-induced cytotoxicity. Inhibition of glucosylceramide synthesis may be a means of circumventing drug resistance by enhancing signaling through a cell-death pathway.

Activation of the endothelial nitric-oxide synthase by tumor necrosis factor-alpha. A novel feedback mechanism regulating cell death

Cell death via apoptosis induced by tumor necrosis factor-alpha (TNF-alpha) plays an important role in many physiological and pathological conditions. The signal transduction pathway activated by this cytokine is known to be regulated by several intracellular messengers. In particular, in many systems nitric oxide (NO) has been shown to protect cells from TNF-alpha-induced apoptosis. However, whether NO can be generated by the cytokine to down-regulate its own apoptotic program has never been studied. We have addressed this question in HeLa Tet-off cell clones stably transfected with the endothelial NO synthase under a tetracycline-responsive promoter. Endothelial NO synthase, induced about 100-fold in these cells by removal of the antibiotic, retained the characteristics of the native enzyme of endothelial cells, both in terms of intracellular localization and functional activity. Expression of the endothelial NO synthase was sufficient to protect from TNF-alpha-induced apoptosis. This protection was mediated by the generation of NO. TNF-alpha itself stimulated endothelial NO synthase activity to generate NO through a pathway involving its lipid messenger, ceramide. Our results identify a novel mechanism of regulation of a signal transduction pathway activated by death receptors and suggest that NO may constitute a built-in mechanism by which TNF-alpha controls its own apoptotic program.

Ceramide glycosylation potentiates cellular multidrug resistance

Ceramide glycosylation, through glucosylceramide synthase (GCS), allows cellular escape from ceramide-induced programmed cell death. This glycosylation event confers cancer cell resistance to cytotoxic anticancer agents [Liu, Y. Y., Han, T. Y., Giuliano, A. E., and M. C. Cabot. (1999) J. Biol. Chem. 274, 1140-1146]. We previously found that glucosylceramide, the glycosylated form of ceramide, accumulates in adriamycin-resistant breast carcinoma cells, in vinblastine-resistant epithelioid carcinoma cells, and in tumor specimens from patients showing poor response to chemotherapy. Here we show that multidrug resistance can be increased over baseline and then totally reversed in human breast cancer cells by GCS gene targeting. In adriamycin-resistant MCF-7-AdrR cells, transfection of GCS upgraded multidrug resistance, whereas transfection of GCS antisense markedly restored cellular sensitivity to anthracyclines, Vinca alkaloids, taxanes, and other anticancer drugs. Sensitivity to the various drugs by GCS antisense transfection increased 7- to 240-fold and was consistent with the resumption of ceramide-caspase-apoptotic signaling. GCS targeting had little influence on cellular sensitivity to either 5-FU or cisplatin, nor did it modify P-glycoprotein expression or rhodamine-123 efflux. GCS antisense transfection did enhance rhodamine-123 uptake compared with parent MCF-7-AdrR cells. This study reveals that GCS is a novel mechanism of multidrug resistance and positions GCS antisense as an innovative force to overcome multidrug resistance in cancer chemotherapy.

Ceramide dissociates 3'-phosphoinositide production from pleckstrin homology domain translocation

Numerous hormones, cytokines and transforming oncogenes activate phosphoinositide 3-kinase (PI-3K), a lipid kinase that initiates signal transduction cascades regulating cellular proliferation, survival, protein synthesis and glucose metabolism. PI-3K catalyses the production of the 3'-phosphoinositides PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3), which recruit downstream effector enzymes to the membrane via their pleckstrin homology (PH) domains. Recent studies have indicated that another signalling lipid, the sphingolipid ceramide, inhibits several PI-3K-dependent events, including insulin-stimulated glucose uptake and growth-factor-stimulated cell survival. Here we show that ceramide analogues specifically prevent the recruitment of the PtdIns(3,4,5)P(3)-binding proteins Akt/protein kinase B (PKB) or the general receptor for phosphoinositides-1 (GRP1). Specifically, the short-chain ceramide derivative C2-ceramide inhibited the platelet-derived growth factor (PDGF)-stimulated translocation of full-length Akt/PKB, as well as truncated proteins encoding only the PH domains of Akt/PKB or GRP1. C2-ceramide did not alter the membrane localization of the PH domain for phospholipase Cdelta, which preferentially binds PtdIns(4,5)P(2), nor did it affect the PDGF-stimulated production of PtdIns(3,4)P(2) or PtdIns(3,4,5)P(3). Interestingly, a glucosylceramide synthase inhibitor, 1-phenyl-2-decanoylamino-3-morpholinopropan-1-ol (PDMP), shown previously to increase intracellular ceramide concentrations without affecting PI-3K [Rani, Abe, Chang, Rosenzweig, Saltiel, Radin and Shayman (1995) J. Biol. Chem. 270, 2859-2867], recapitulated the inhibitory effects of C2-ceramide on PDGF-stimulated Akt/PKB phosphorylation. These studies indicate that ceramide prevents the translocation of certain PtdIns(3,4,5)P(3)-binding proteins, despite the presence of a full complement of PtdIns(3,4)P(2) or PtdIns(3,4,5)P(3). Furthermore, these findings suggest a mechanism by which stimuli that induce ceramide synthesis could negate the fundamental signalling pathways initiated by PI-3K.

Regulation of cyclin-dependent kinase 2 activity by ceramide

Cyclin-dependent kinases have been implicated in the inactivation of retinoblastoma (Rb) protein and cell cycle progression. Recent studies have demonstrated that the lipid molecule ceramide is able to induce Rb hypophosphorylation leading to growth arrest and cellular senescence. In this study, we examined the underlying mechanisms of Rb hypophosphorylation and cell cycle progression utilizing the antiproliferative molecule ceramide. C6-Ceramide induced a G0/G1 arrest of the cell cycle in WI38 human diploid fibroblasts. Employing immunoprecipitation kinase assays, we found that ceramide specifically inhibited cyclin-dependent kinase CDK2, with a mild effect on CDC2 and significantly less effect on CDK4. The effect of ceramide was specific such that C6-dihydroceramide was not effective. Ceramide did not directly inhibit CDK2 in vitro but caused activation of p21, a major class of CDK-inhibitory proteins, and led to a greater association of p21 to CDK2. Using purified protein phosphatases, we showed that ceramide activated both protein phosphatase 1 and protein phosphatase 2A activities specific for CDK2 in vitro. Further, calyculin A and okadaic acid, both potent protein phosphatase inhibitors, together almost completely reversed the effects of ceramide on CDK2 inhibition. Taken together, these results demonstrate a dual mechanism by which ceramide inhibits the cell cycle. Ceramide causes an increase in p21 association with CDK2 and through activation of protein phosphatases selectively regulates CDK2. These events may lead to activation of Rb protein and subsequent cell cycle arrest.

Stemming the tide: glycosphingolipid synthesis inhibitors as therapy for storage diseases

Glycosphingolipids (GSLs) are plasma membrane components of every eukaryotic cell. They are composed of a hydrophobic ceramide moiety linked to a glycan chain of variable length and structure. Once thought to be relatively inert, GSLs have now been implicated in a variety of biological processes. Recent studies of animals rendered genetically deficient in various classes of GSLs have demonstrated that these molecules are important for embryonic differentiation and development as well as central nervous system function. A family of extremely severe diseases is caused by inherited defects in the lysosomal degradation pathway of GSLs. In many of these disorders GSLs accumulate in cells, particularly neurons, causing neurodegeneration and a shortened life span. No effective treatment exists for most of these diseases and little is understood about the mechanisms of pathogenesis. This review will discuss the development of a new approach to the treatment of GSL storage disorders that targets the major synthesis pathway of GSLs to stem their cellular accumulation.

Cellular gangliosides promote growth factor-induced proliferation of fibroblasts

Cell surface gangliosides have been proposed as modulators of transmembrane signaling. In this study, we used two complementary approaches to investigate the function of cellular gangliosides in the response of mammalian fibroblasts to growth factors. First, inhibition of glucosyl ceramide synthase by a new specific inhibitor of d-l-threo-1-phenyl-2-hexadecanoylamino-3 -pyrrolidino-1-propanol-HC l (glucosylceramide synthase), which depletes cellular gangliosides at a concentration of 1 microm without causing an increase in ceramide levels, blocked epidermal growth factor-stimulated proliferation of fibroblasts. Similarly, responses to several other growth factors that activate receptor tyrosine kinases, including fibroblast growth factor, insulin-like growth factor-I, and platelet-derived growth factor, were inhibited by 50-100%. Conversely, enrichment of cellular gangliosides by preincubation of the mouse and human fibroblasts with exogenously added gangliosides enhanced growth factor-elicited cell proliferation. Novel findings of this study, distinguishing it from previous similar studies, include differential effects of preincubation versus continuous incubation of cells with gangliosides on growth factor-dependent cell proliferation and the growth factor-like action of NeuNAc alpha 2-3Gal beta 1-3GalNAc beta 1-4(NeuNAc alpha 2-3)Gal beta 1-4Glc beta 1-1Cer when cells are pretreated with the ganglioside.

The effect of 50 Hz magnetic field on GCSmRNA expression in lymphoma B cell by mRNA differential display

Magnetic fields (MFs) of various characteristics can lead to plethora effects in biological system. From a molecular point of view, we hypothesized that there must be a fundamental difference in gene expression between the MF exposed and the unexposed cell. To identify the classes of genes that are regulated, 0.8 mT 50 Hz MF-induced changes in gene expression were examined in a Daudi cell culture using differential display and reverse transcriptase-polymerase chain reaction. A candidate cDNA (signatured as MF-CB) that was observed in the sham-exposed but not in MF-exposed cultures was recovered and reamplified. After verification by Northern blot, the cDNA was cloned and sequenced. It was found that 254-base pair of 5'-end MF-CB cDNA clone was identical to gcs in open reading frame (ORF) range. Based on the preliminarily sequence, the prolonged length of 5'-end MF-CB cDNA was obtained by PCR amplification and its sequence analysis showed the same results as its original fragment. In order to further determine whether MF-CB cDNA is from gcs, two Northern blots were probed with gcs and MF-CB cDNA, respectively, and the data revealed signals of the same size and expression pattern on the two probe filters, which demonstrated that MF-CB is an EST (expression sequence tag) of gcs. gcs is a gene, identified recently (GenBank accession number D89866), encoding ceramide glucosyltransferase (GCS), which has been implicated as a causal element in human cell growth and differentiation. In an additional experiment, time-dependent changes in the transcription of gcs induced by 0.8 mT MF were observed by Northern blot with a sharp and reproducible inhibition effect after 20 min exposure and a reduction after 20-24 h exposure. The study demonstrates for the first time that 50 Hz MF can lead to changes in gcs transcription, which provides a new clue to elucidate the mechanism by which MF influence cell growth and differentiation.

Reduction of globotriaosylceramide in Fabry disease mice by substrate deprivation

We used a potent inhibitor of glucosylceramide synthase to test whether substrate deprivation could lower globotriaosylceramide levels in alpha-galactosidase A (alpha-gal A) knockout mice, a model of Fabry disease. C57BL/6 mice treated twice daily for 3 days with D-threo-1-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidi no-propanol (D-t-EtDO-P4) showed a concentration-dependent decrement in glucosylceramide levels in kidney, liver, and spleen. A single intraperitoneal injection of D-t-EtDO-P4 resulted in a 55% reduction in renal glucosylceramide, consistent with rapid renal glucosylceramide metabolism. A concentration-dependent decrement in renal and hepatic globotriaosylceramide levels was observed in alpha-Gal A(-) males treated for 4 weeks with D-t-EtDO-P4. When 8-week-old alpha-Gal A(-) males were treated for 8 weeks with 10 mg/kg twice daily, renal globotriaosylceramide fell to below starting levels, consistent with an alpha-galactosidase A-independent salvage pathway for globotriaosylceramide degradation. Complications observed with another glucosylceramide synthase inhibitor, N-butyldeoxynojirimycin, including weight loss and acellularity of lymphatic organs, were not observed with D-t-EtDO-P4. These data suggest that Fabry disease may be amenable to substrate deprivation therapy.

Nitric oxide inhibits tumor necrosis factor-alpha-induced apoptosis by reducing the generation of ceramide

Apoptosis triggered by death receptors proceeds after defined signal-transduction pathways. Whether signaling at the receptor level is regulated by intracellular messengers is still unknown. We have investigated the role of two messengers, ceramide and nitric oxide (NO), on the apoptotic pathway activated in human monocytic U937 cells by tumor necrosis factor-alpha (TNF-alpha) working at its p55 receptor. Two transduction events, the receptor recruitment of the adapter protein, TRADD, and the activation of the initiator caspase, caspase 8, were investigated. When administered alone, neither of the messengers had any effect on these events. In combination with TNF-alpha, however, ceramide potentiated, whereas NO inhibited, TNF-alpha-induced TRADD recruitment and caspase 8 activity. The effect of NO, which was cGMP-dependent, was due to inhibition of the TNF-alpha-induced generation of ceramide. Our results identify a mechanism of regulation of a signal-transduction pathway activated by death receptors.

Cloning and expression of glycolipid transfer protein from bovine and porcine brain

Glycolipid transfer protein (GLTP) is a small (23-24 kDa), basic protein (pI congruent with 9.0) that accelerates the intermembrane transfer of various glycolipids. Here, we report the first cloning of cDNAs that encode the bovine and porcine GLTPs. The cDNA open reading frame for bovine GLTP was constructed by bridge-overlapping extension polymerase chain reaction (PCR) after obtaining partial coding cDNA clones by hot start, seminested, and rapid amplification of cDNA ends-PCR. The cDNA open reading frame for porcine GLTP was constructed by reverse transcriptase-PCR. The encoded amino acid sequences in the full-length bovine and porcine cDNAs were identical, consisting of 209 amino acid residues, and were nearly the same as the published sequence determined by Edman degradation. The cDNA encoded one additional amino acid at the N terminus (methionine), arginine at positions 10 and 200 instead of lysine, and threonine at position 65 instead of alanine. Expression of GLTP-cDNA in Escherichia coli using pGEX-6P-1 vector resulted in glutathione S-transferase (GST)-GLTP fusion protein. Regulation of growth and induction conditions led to approximately 50% of expressed fusion protein being soluble and active. Proteolytic cleavage of GST-GLTP fusion protein (bound to GST-Sepharose) and affinity purification resulted in fully active GLTP. Northern blot analyses of bovine tissues showed a single transcript of approximately 2.2 kilobases and the following hierarchy of mRNA levels: cerebrum > kidney > spleen congruent with lung congruent with cerebellum > liver > heart muscle. Reverse transcriptase-PCR analyses of mRNA levels supported the Northern blot results.

Ceramide induces expression of the senescence histochemical marker, beta-galactosidase, in human fibroblasts

We recently showed that ceramide is elevated in senescence and that when administered to low-passage cells induces biochemical changes characteristic of senescence. The in situ histochemical marker beta-galactosidase (beta-Gal) has provided an important tool in the study of cellular senescence. We investigated the ability of ceramide to induce the expression of beta-Gal and correlated this with cell proliferation. We find that D-e-C6-ceramide, induces the expression of acidic beta-Gal in fetal lung-derived Wi-38 human diploid fibroblasts. Our results show that this induction is: (1) time and concentration dependent; and (2) reversible upon ceramide removal. We also find that concomitant with the onset of beta-Gal staining, DNA synthesis is blocked. These conditions are reversible. The induction of beta-Gal expression is specific to C6-ceramide. We discuss a potential role of beta-Gal in the regulation of senescence. Although signal transduction of senescence is still not fully understood, this new evidence strengthens the hypothesis that ceramide plays a key role in signaling down stream biochemical changes in cellular senescence.

Glycosphingolipid depletion in fabry disease lymphoblasts with potent inhibitors of glucosylceramide synthase

BACKGROUND

Fabry disease is an inherited X-linked disorder resulting in the loss of activity of the lysosomal hydrolase alpha-galactosidase A and causing the clinical manifestations of renal failure, cerebral vascular disease, and myocardial infarction. The phenotypic expression of this disorder is manifest by the accumulation of glycosphingolipids containing alpha-galactosyl linkages, most prominently globotriaosylceramide.

METHODS

Based on quantitative structure activity studies, we recently reported two newly designed glucosylceramide synthase inhibitors based on 1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (P4). These inhibitors, 4'-hydroxy-P4 and ethylenedioxy-P4, were evaluated for their ability to deplete globotriaosylceramide and other glucosylceramide-based lipids in Fabry lymphocytes and were compared with N-butyldeoxynojirimycin, another reported glucosylceramide synthase inhibitor.

RESULTS

Concentrations as low as 10 nmol/L of 4'-hydroxy-P4 and ethylenedioxy-P4 resulted in 70 and 80% depletion, respectively, of globotriaosylceramide, with maximal depletion occurring at three days of treatment. There was no impairment of cell growth. In contrast, N-butyldeoxynojirimycin only minimally lowered globotriaosylceramide levels, even at concentrations as high as 10 micromol/L. Globotriaosylceramide depletion was confirmed by the loss of binding of FITC-conjugated verotoxin B subunit to the lymphoblasts.

CONCLUSIONS

These findings suggest that selective glucosylceramide synthase inhibitors are highly effective in the depletion of globotriaosylceramide from Fabry cell lines. We suggest that these compounds have potential therapeutic utility in the treatment of Fabry disease.

Caveolar structure and protein sorting are maintained in NIH 3T3 cells independent of glycosphingolipid depletion

Glycosphingolipids have been proposed to be critical components of clustered lipids within cell membranes that serve as rafts for the attachment and sorting of proteins to the cell membrane. Density gradient centrifugation was used to isolate and to ascertain the lipid composition of caveolin-enriched membranes. These membranes demonstrated a significant enrichment of sphingolipids and cholesterol containing up to 20 and 30%, respectively, of the cellular glucosylceramide and lactosylceramide. A specific inhibitor of glucosylceramide synthase, d-threo-1-phenyl-2-palmitoyl-3-pyrrolidino-propanol, was used to test the hypothesis that glycosphingolipids are required for the sorting of proteins to caveolae. When NIH 3T3 cells were depleted of their glucosylceramide based glycosphingolipid mass, the caveolar structure remained intact as determined by electron microscopy and confocal microscopy. The caveolar proteins caveolin and annexin II sorted normally to caveolae, as determined by immunoblotting and confocal microscopy. When the GPI-linked protein B61 was inducibly expressed in these cells, sorting to caveolar membranes occurred normally, even in the presence of glucosylceramide depletion. These observations suggest that protein sorting to caveolae in fibroblasts occurs independently of glycosphingolipid synthesis.

Glucosylceramide synthase: assay and properties

Glucosylceramide synthesis is a key step in the formation of most mammalian glycosphingolipids. The expanding number of cellular functions that may be glycosphinolipid dependent and the identification of this glucosylceramide synthase as a potential therapeutic target for several sphingolipid storage disorders necessitate the availability of a reliable assay for glucosylceramide synthase. Coupled with the recent sequencing of this enzyme, the liposome-based assay utilizing a single extraction step should aid in the understanding of this critical early pathway in glycosphingolipid formation.

Preferential killing of multidrug-resistant KB cells by inhibitors of glucosylceramide synthase

This study has compared the preferential killing of three multidrug-resistant (MDR) KB cell lines, KB-C1, KB-A1 and KB-V1 by two inhibitors of glucosylceramide synthase, 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and 1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol (PPPP), to the killing produced by these compounds in the drug-sensitive cell line, KB-3-1. Both of the inhibitors caused much greater induction of apoptosis in each of the three MDR cell lines than in the drug-sensitive cell line, as judged by morphological assay and confirmed by poly-(ADP-ribose)-polymerase cleavage. The highest level of apoptosis was produced following 24-h exposure to 5 microM PPPP. This treatment produced 75.8 (+/- 7.1)%, 73.6 (+/- 9.8)% and 75.3 (+/- 6.4)% apoptotic cells in the three MDR cell lines respectively, compared to 19.0 (+/- 9.8)% in the drug-sensitive cell line. A reduction in glucosylceramide level following inhibitor treatment occurred in KB-3-1 cells as well as in the MDR cell lines, suggesting that the increased apoptotic response in the MDR cells reflected a different downstream response to changes in the levels of this lipid in these cells compared to that in the drug-sensitive cells. These results suggest that the manipulation of glucosylceramide levels may be a fruitful way of causing the preferential killing of MDR cells in vitro and possibly in vivo.

Multidrug resistance modulators and doxorubicin synergize to elevate ceramide levels and elicit apoptosis in drug-resistant cancer cells

BACKGROUND

To provide insight for the development of more effective clinical agents, the authors attempted to elucidate the mechanisms of action of multidrug resistance (MDR) modulators. Previously, the authors found that MDR modulators blocked the conversion of ceramide to glucosylceramide in MDR cells, thereby enhancing cytotoxicity. Because ceramide is a critical component of the apoptosis signaling cascade, the current study examined the impact of therapy using agents that elicit ceramide formation combined with agents that block ceramide glycosylation.

METHODS

Doxorubicin-resistant human breast carcinoma cells (MCF-7-AdrR) were treated with either doxorubicin, tamoxifen, cyclosporine A, or the cyclosporine A analog SDZ PSC 833 (PSC 833) or with combinations thereof, and ceramide and glucosylceramide metabolisms were measured by cell radiolabeling. Cell viability was quantitated spectrophotometrically and apoptosis was evaluated analyzing DNA integrity by gel electrophoresis.

RESULTS

Whereas cyclosporine A blocked the generation of glucosylceramide in MCF-7-AdrR cells, a chemical cousin, PSC 833, elicited a 3-fold increase in glucosylceramide and a 5-fold increase in ceramide levels at 24 hours. The PSC 833 response was time-dependent(as early as 30 minutes) and dose-dependent (as low as 0.1 microM). The appearance of ceramide foreran the generation of glucosylceramide. Sphingomyelin levels were not decreased in response to PSC 833; however, Fumonisin B1, a ceramide synthase inhibitor, blocked PSC 833-induced ceramide generation. Adding tamoxifen, which blocks ceramide glycosylation, to the PSC 833 regimen boosted ceramide levels 11-fold over controls and caused DNA fragmentation. A 3-component regimen comprised of tamoxifen, doxorubicin, and PSC 833 increased ceramide levels 26-fold and brought cell viability to zero.

CONCLUSIONS

These results demonstrate that MDR modulators can be used separately, in combination, or in conjunction with chemotherapy at clinically relevant concentrations to manipulate cellular ceramide levels and restore sensitivity in the drug resistant setting. As such, this represents a new direction in the treatment of cancer.

Improved inhibitors of glucosylceramide synthase

Previous work has led to the identification of inhibitors of glucosylceramide synthase, the enzyme catalyzing the first glycosylation step in the synthesis of glucosylceramide-based glycosphingolipids. These inhibitors have two identified sites of action: the inhibition of glucosylceramide synthase, resulting in the depletion of cellular glycosphingolipids, and the inhibition of 1-O-acylceramide synthase, resulting in the elevation of cell ceramide levels. A new series of glucosylceramide synthase inhibitors based on substitutions in the phenyl ring of a parent compound, 1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (P4), was made. For substitutions of single functional groups, the potency of these inhibitors in blocking glucosylceramide synthase was primarily dependent upon the hydrophobic and electronic properties of the substituents. An exponential relationship was found between the IC50 of each inhibitor and the sum of derived hydrophobic (pi) and electronic (sigma) parameters. This relationship demonstrated that substitutions that increased the electron-donating characteristics and decreased the lipophilic characteristics of the homologues enhanced the potency of these compounds in blocking glucosylceramide formation. A novel compound was subsequently designed and observed to be even more active in blocking glucosylceramide formation. This compound, D-threo-4'-hydroxy-P4, inhibited glucosylceramide synthase at an IC50 of 90 nM. In addition, a series of dioxane substitutions was designed and tested. These included 3',4'-methylenedioxyphenyl-, 3',4'-ethylenedioxyphenyl-, and 3'4'-trimethylenedioxyphenyl-substituted homologues. D-threo-3', 4'-Ethylenedioxy-P4-inhibited glucosylceramide synthase was comparably active to the p-hydroxy homologue. 4'-Hydroxy-P4 and ethylenedioxy-P4 blocked glucosylceramide synthase activity at concentrations that had little effect on 1-O-acylceramide synthase activity. These novel inhibitors resulted in the inhibition of glycosphingolipid synthesis in cultured cells at concentrations that did not significantly raise intracellular ceramide levels or inhibit cell growth.