Metabolism of D-[3H]threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, an inhibitor of glucosylceramide synthesis, and the synergistic action of an inhibitor of microsomal monooxygenase

Colocalization of receptors for Shiga toxins with lipid rafts in primary human renal glomerular endothelial cells and influence of D-PDMP on synthesis and distribution of glycosphingolipid receptors

Damage of human renal glomerular endothelial cells (HRGECs) of the kidney represents the linchpin in the pathogenesis of the hemolytic uremic syndrome caused by Shiga toxins of enterohemorrhagic Escherichia coli (EHEC). We performed a comprehensive structural analysis of the Stx-receptor glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer, Galα4Galβ4Glcβ1Cer) and globotetraosylceramide (Gb4Cer, GalNAcβ3Galα4Galβ4Glcβ1Cer) and their distribution in lipid raft analog detergent-resistant membranes (DRMs) and nonDRMs prepared from primary HRGECs. Predominant receptor lipoforms were Gb3Cer and Gb4Cer with Cer (d18:1, C16:0), Cer (d18:1, C22:0) and Cer (d18:1, C24:1/C24:0). Stx-receptor GSLs co-distribute with sphingomyelin (SM) and cholesterol as well as flotillin-2 in DRMs, representing the liquid-ordered membrane phase and indicating lipid raft association. Lyso-phosphatidylcholine (lyso-PC) was identified as a nonDRM marker phospholipid of the liquid-disordered membrane phase. Exposure of primary HRGECs to the ceramide analogon d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) reduced total Gb3Cer and Gb4Cer content, roughly calculated from two biological replicates, down to half and quarter of its primordial content, respectively, but strengthened their prevalence and cholesterol preponderance in DRMs. At the same time, the distribution of PC, SM and lyso-PC to subcellular membrane fractions remained unaffected by D-PDMP treatment. Defining the GSL composition and precise microdomain structures of primary HRGECs may help to develop novel therapeutic options to combat life-threatening EHEC infections.

The novel HDAC inhibitor AR-42-induced anti-colon cancer cell activity is associated with ceramide production

In the current study, we investigated the potential activity of AR-42, a novel histone deacetylase (HDAC) inhibitor, against colon cancer cells. Our in vitro results showed that AR-42 induced ceramide production, exerted potent anti-proliferative and pro-apoptotic activities in established (SW-620 and HCT-116 lines) and primary human colon cancer cells. Exogenously-added sphingosine 1-phosphate (S1P) suppressed AR-42-induced activity, yet a cell-permeable ceramide (C4) facilitated AR-42-induced cytotoxicity against colon cancer cells. In addition, AR-42-induced ceramide production and anti-colon cancer cell activity were inhibited by the ceramide synthase inhibitor fumonisin B1, but were exacerbated by PDMP, which is a ceramide glucosylation inhibitor. In vivo, oral administration of a single dose of AR-42 dramatically inhibited SW-620 xenograft growth in severe combined immunodeficient (SCID) mice, without inducing overt toxicities. Together, these results show that AR-42 dramatically inhibits colon cancer cell proliferation in vitro and in vivo, and ceramide production might be the key mechanism responsible for its actions.

The glucosylceramide synthase inhibitor PDMP sensitizes pancreatic cancer cells to MEK/ERK inhibitor AZD-6244

Here we show that d,l-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), a glycosphingolipid biosynthesis inhibitor, increases the sensitivity of pancreatic cancer cells to the novel MEK-ERK inhibitor AZD-6244. AZD-6244 and PDMP co-administration induced massive pancreatic cancer cell death and apoptosis, more potently than either drug alone. We discovered that AZD-6244 induced ceramide production in pancreatic cancer cells, yet the excess ceramide was metabolically removed in the long-term (24-48h). PDMP facilitated AZD-6244-induced ceramide production, and ceramide level remained elevated up to 48h. Meanwhile, exogenously-added cell-permeable short chain ceramide (C2) similarly sensitized AZD-6244's activity, the two caused substantial pancreatic cancer cell death and apoptosis. At the molecular level, PDMP and AZD-6244 co-treatment inactivated ERK1/2 and AKT-mTOR signalings simultaneously in pancreatic cancer cells, while either agent alone only affected one signaling. In summary, PDMP significantly increased the sensitivity of AZD-6244 in pancreatic cancer cells. This appears to involve a sustained ceramide production as well as concurrent block of ERK and AKT-mTOR signalings.

Glucosylceramide transferase activity is critical for encystation and viable cyst production by an intestinal protozoan, Giardia lamblia

The production of viable cysts by Giardia is essential for its survival in the environment and for spreading the infection via contaminated food and water. The hallmark of cyst production (also known as encystation) is the biogenesis of encystation-specific vesicles (ESVs) that transport cyst wall proteins to the plasma membrane of the trophozoite before laying down the protective cyst wall. However, the molecules that regulate ESV biogenesis and maintain cyst viability have never before been identified. Here, we report that giardial glucosylceramide transferase-1 (gGlcT1), an enzyme of sphingolipid biosynthesis, plays a key role in ESV biogenesis and maintaining cyst viability. We find that overexpression of this enzyme induced the formation of aggregated/enlarged ESVs and generated clustered cysts with reduced viability. The silencing of gGlcT1 synthesis by antisense morpholino oligonucleotide abolished ESV production and generated mostly nonviable cysts. Interestingly, when gGlcT1-overexpressed Giardia was transfected with anti-gGlcT1 morpholino, the enzyme activity, vesicle biogenesis, and cyst viability returned to normal, suggesting that the regulated expression of gGlcT1 is important for encystation and viable cyst production. Furthermore, the overexpression of gGlcT1 increased the influx of membrane lipids and fatty acids without altering the fluidity of plasma membranes, indicating that the expression of gGlcT1 activity is linked to lipid internalization and maintaining the overall lipid balance in this parasite. Taken together, our results suggest that gGlcT1 is a key player of ESV biogenesis and cyst viability and therefore could be targeted for developing new anti-giardial therapies.

Sphingosine kinase-1 inhibition sensitizes curcumin-induced growth inhibition and apoptosis in ovarian cancer cells

Recent published studies suggest that increasing levels of ceramides enhance the chemo-sensitivity of curcumin. Using in vitro approaches, we analyzed the impact of sphingosine kinase-1 (SphK-1) inhibition on ceramide production, and evaluated SphK1 inhibitor II (SKI-II) as a potential curcumin chemo-sensitizer in ovarian cancer cells. We found that SphK1 is overexpressed in ovarian cancer patients' tumor tissues and in cultured ovarian cancer cell lines. Inhibition of SphK1 by SKI-II or by RNA interference (RNAi) knockdown dramatically enhanced curcumin-induced apoptosis and growth inhibition in ovarian cancer cells. SKI-II facilitated curcumin-induced ceramide production, p38 activation and Akt inhibition. Inhibition of p38 by the pharmacological inhibitor (SB 203580), a dominant-negative expression vector, or by RNAi diminished curcumin and SKI-II co-administration-induced ovarian cancer cell apoptosis. In addition, restoring Akt activation introducing a constitutively active Akt, or inhibiting ceramide production by fumonisin B1 also inhibited the curcumin plus SKI-II co-administration-induced in vitro anti-ovarian cancer effect, suggesting that ceramide accumulation, p38 activation and Akt inhibition are downstream effectors. Our findings suggest that low, well-tolerated doses of SKI-II may offer significant improvement to the clinical curcumin treatment of ovarian cancer.

1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) facilitates curcumin-induced melanoma cell apoptosis by enhancing ceramide accumulation, JNK activation, and inhibiting PI3K/AKT activation

The majority of metastatic melanomas are resistant to different chemotherapeutic agents, consequently, the search for novel anti-melanoma agents and adjuvant is urgent. Here, we found that 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), an inhibitor of glycosphingolipid biosynthesis, enhanced curcumin-induced cell growth inhibition and apoptosis in two melanoma cell lines (WM-115 and B16). PDMP facilitated curcumin-induced ceramide accumulation; the latter contributed to melanoma cell apoptosis. PDMP also dramatically enhanced curcumin-induced c-Jun N-terminal kinase activation, which was important to melanoma cell apoptosis. Meanwhile, curcumin plus PDMP treatment largely inhibited the activation of pro-survival PI3K/AKT signal pathway. In conclusion, PDMP-sensitized curcumin-induced melanoma cell growth inhibition and apoptosis in vitro due to changes of multiple signal events. Combining PDMP with curcumin may represent a new therapeutic intervention against melanoma.

Designing anticancer drugs via the achilles heel: ceramide, allylic ketones, and mitochondria

Published reports are reviewed as the basis of a proposal that an effective antineoplastic drug should contain several features: (a) resemblance to the natural lipid, ceramide; (b) an allylic alcohol and/or allylic ketone moiety; (c) a hydroxyl and/or a nitrogen atom near the allylic group; (d) conjugated double bonds as part of the allylic region. The drug should produce reactive oxygen species in tumor mitochondria, stimulate the generation of ceramide in the tumor, and condense with mitochondrial glutathione. It is pointed out that some antibiotics with these features are also active against cancer cells; perhaps anticancer drugs with these features will prove useful as antibiotics. Common problems in working with lipoidal substances are discussed.

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.

Differential effects of glycolipid biosynthesis inhibitors on ceramide-induced cell death in neuroblastoma cells

An in vitro model of Gaucher's disease in murine neuroblastoma x rat glioma NG108-15 cells was used to investigate the physiological effects of two specific inhibitors of glucosylceramide synthase, d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (d,l-PDMP) and N-butyldeoxynojirimycin (NB-DNJ), which have been suggested as agents for treatment of glycolipid storage disorders. Incubation of NG108-15 cells with conduritol-B-epoxide, a covalent inhibitor of glucosylceramidase, raised the intracellular concentration of glucosylceramide (GC) by more than fourfold, indicating a glycolipid composition equivalent to that of Gaucher's cells. The level of GC was decreased, and the cells were depleted of gangliosides by postincubation with d,l-PDMP or NB-DNJ. Treatment with d,l-PDMP, but not with NB-DNJ, resulted in a dose-dependent reduction of the growth rate and eventually caused cell death in NG108-15 cells on reaching confluency. An in situ detection assay using terminal nucleotidyltransferase indicated that cell degeneration was accompanied by apoptosis. Lipid analysis by high-performance TLC revealed that on incubation with d,l-PDMP, but not with NB-DNJ, the concentration of endogenous ceramide was elevated by threefold. Ceramide elevation and apoptosis were also observed when NG108-15 cells were incubated with daunorubicin, which was previously reported to induce programmed cell death by stimulation of ceramide synthesis. Structural characterization by HPLC and subsequent laser desorption mass spectrometry revealed that the endogenous ceramide contained fatty acids with chain lengths ranging from C14:0 to C24:0. The results indicate that elevation of levels of these ceramide species by incubation with d,l-PDMP or daunorubicin induces programmed cell death in NG108-15 cells. Because ceramide accumulation and cell death were not observed on incubation with NB-DNJ, its use is suggested to be less toxic than that of d,l-PDMP for treatment of Gaucher's disease and other sphingolipid storage disorders.

Up-regulation of ganglioside biosynthesis, functional synapse formation, and memory retention by a synthetic ceramide analog (L-PDMP)

To address the role of brain gangliosides in synaptic activity, the ceramide analogs, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) and its enantiomer, L-PDMP, were used to inhibit and stimulate ganglioside biosynthesis in cultured cortical neurons. Prolonged treatment with both PDMP isomers exhibited opposite effects on functional synapse formation measured by spontaneous synchronized oscillatory activity of intracellular Ca2+ between the neurons: suppression by D-PDMP and facilitation by L-PDMP. Up-regulation of synaptic activity by L-PDMP could be correlated with the slow but robust stimulation of ganglioside biosynthesis through activating GM3, GD3 and GQ1b synthases. In a similar time course, the activity of p42 mitogen-activated protein kinase was also enhanced by L-PDMP. To evaluate the efficacy of this drug in long-term memory, rats were trained for 2 weeks using an 8-arm radial maze task, and then forebrain ischemia was induced by 4-vessel occlusion. Treatment with L-PDMP starting 24 hours after the transient ischemia ameliorated the deficit of a well-learned spatial memory, demonstrating the potential therapeutic intervention of the ceramide analog for neurodegenerative disorders.

Treatment of Gaucher disease with an enzyme inhibitor

The hypothesis is offered predicting that Gaucher patients could be treated with a drug that slows the synthesis of glucosylceramide, the lipid that accumulates in this disorder. The present therapeutic approach involves augmenting the defective enzyme, glucosylceramide beta-glucosidase, with exogenous beta-glucosidase isolated from human tissue. This spectacularly expensive mode of treatment should be replaceable with a suitable enzyme inhibitor that simply slows formation of the lipid and matches the rate of synthesis with the rate of the defective, slowly working beta-glucosidase. Several drugs that possess this ability are available, the best known of which is 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), a designer inhibitor that resembles the synthase's substrate and product. PDMP has been found to be effective in mice, rats, fish, and a wide variety of cultured cells. Its use, at suitable dosages, seems to be harmless, although long-term tests have not been made. The lack of suitable animal models of Gaucher disease has made it difficult to test the hypothesis adequately, but PDMP does rapidly lower the levels of glucosylceramide in normal animal tissues and the animals evidently do well with the lowered levels of glucosylceramide and its more complex glycolipid metabolites.

Induction of glucosylceramide synthase by synthase inhibitors and ceramide

Glucosylceramide (GlcCer) synthase acts on the sphingolipid, ceramide, to transer a glucose moiety from UDP-glc, thus forming the first member of a large family of glucosphingolipids. Two inhibitors of the enzyme, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-threo-PDMP) and N-butyldeoxynojirimycin (NBDN), have been found to induce an elevated level of the synthase in MDCK cells. In cells treated with 20 muM PDMP, then assayed for synthase activity under conditions in which the absorbed PDMP was partially diluted out, the assay showed that the enzyme's specific activity had risen considerably in only 1 h and reached a maximum of about three times the control activity within 6 h. Both cycloheximide and actinomycin D, inhibitors of translational and transcriptional protein synthesis, caused much of the synthase activity to disappear in 6 h, presumably because of normal catabolic destruction. However, simultaneous inclusion of PDMP or NBDN in the cell medium slowed the rate of synthase disappearance. L-Cycloserine, which blocked the synthesis of ceramide, nevertheless allowed PDMP to elevate the synthase activity. Thus the inductive effect appears to be due, in part at least, to resistance of the enzyme-inhibitor complex to the normal process of enzyme degradation. Two other inhibitors of GlcCer synthase, more active than PDMP, did not produce detectable induction because they could not be dissociated from the enzyme during the cell washing and diluting steps. Agents that produced a large increase in endogenous cell ceramide level (DL-erythro-PDMP,N-acetylsphingosine, and bacterial sphingomyelinase) also induced an elevated level of GlcCer synthase. The latter two agents did not protect the synthase from catabolism in the presence of cycloheximide. These findings suggest the existence of a second mechanism of enzyme induction, enhanced synthesis of the enzyme due to the increased availability of the enzyme's lipoidal substrate. The possibility is raised that events involving ceramide in cell signalling may be mediated in part by changes in glucosphingolipid levels.

Rationales for cancer chemotherapy with PDMP, a specific inhibitor of glucosylceramide synthase

A proposed weak point in cancer cells is their need to synthesize novel or rare glucosphingolipids. It is further proposed that cancer patients be treated with a drug that slows the synthesis of glucosylceramide, the precursor of a large family of glucosphingolipids. Experimental data are furnished for chemotherapeutic and biochemical effects of PDMP, an analog of glucosylceramide and its precursor, ceramide. Promising results were obtained in the treatment of mice carrying Ehrlich ascites carcinoma cells and rats carrying C6 glioma cells. PDMP was found to be oxidized by cytochrome P-450, but this process could be blocked in vivo with piperonyl butoxide or cimetidine. A high level of blood glucose was found to elevate the size of rat kidneys and their content of UDP-glucose and its product, glucosylceramide. The excessive growth could be blocked by PDMP, which competes with UDP-glc for binding to glucosylceramide synthase. It is suggested that cancer patients be maintained at a low glucose level in order to slow the synthesis of glucosylceramide by tumor cells. Metabolic changes produced by PDMP in cultured cells, besides a rapid deletion of glucosphingolipids, were accumulation of the precursors (ceramide and sphingosine), loss of protein kinase C, and accumulation of diacylglycerol. It is suggested that many of the cellular changes produced by PDMP, such as loss of cell binding, are owing to existence of glucosylceramide-based "islands" floating in the outer cell surface; the islands may contain growth factor receptors and adhesion factors. An inhibitor that blocks sphingolipid synthesis, such as cycloserine, may prove to be a useful adjuvant for therapy with PDMP.