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

Inhibitors of Glucosylceramide Synthase

Glucosylceramide synthase can be targeted by high affinity small molecular weight inhibitors for the study of glycosphingolipid metabolism and function or for the treatment of glycosphingolipid storage disorders, including Gaucher and Fabry disease. This work is exemplified by the discovery and development of eliglustat tartrate, the first stand-alone small chemical entity approved for the treatment of Gaucher disease type 1. The development of inhibitors of glucosylceramide synthase that have utility for either research or clinical purposes begins with a testing funnel for screening candidate inhibitors for activity against this enzyme and for activity in lowering the content of glucosylceramide in intact cells. Two common assays for glucosylceramide synthase, one enzyme based and another cell based, are the focus of this chapter.

Regulation of sphingolipid synthesis by the G1/S transcription factor Swi4

SBF (Swi4/Swi6 Binding Factor) complex is a crucial regulator of G1/S transition in Saccharomyces cerevisiae. Here, we show that SBF complex is required for myriocin resistance, an inhibitor of sphingolipid synthesis. This phenotype was not shared with MBF complex mutants nor with deletion of the Swi4p downstream targets, CLN1/CLN2. Based on data mining results, we selected putative Swi4p targets related to sphingolipid metabolism and studied their gene transcription as well as metabolite levels during progression of the cell cycle. Genes which encode key enzymes for the synthesis of long chain bases (LCBs) and ceramides were periodically transcribed during the mitotic cell cycle, having a peak at G1/S, and required SWI4 for full transcription at this stage. In addition, HPLC-MS/MS data indicated that swi4Δ cells have decreased levels of sphingolipids during progression of the cell cycle, particularly, dihydrosphingosine (DHS), C24-phytoceramides and C24-inositolphosphoryl ceramide (IPC) while it had increased levels of mannosylinositol phosphorylceramide (MIPC). Furthermore, we demonstrated that both inhibition of de novo sphingolipid synthesis by myriocin or SWI4 deletion caused partial arrest at the G2/M phase. Importantly, our lipidomic data demonstrated that the sphingolipid profile of WT cells treated with myriocin resembled that of swi4Δ cells, with lower levels of DHS, IPC and higher levels of MIPC. Taken together, these results show that SBF complex plays an essential role in the regulation of sphingolipid homeostasis, which reflects in the correct progression through the G2/M phase of the cell cycle.

Correction of cilia structure and function alleviates multi-organ pathology in Bardet-Biedl syndrome mice

Bardet–Biedl syndrome (BBS) is a pleiotropic autosomal recessive ciliopathy affecting multiple organs. The development of potential disease-modifying therapy for BBS will require concurrent targeting of multi-systemic manifestations. Here, we show for the first time that monosialodihexosylganglioside accumulates in Bbs2^−/− cilia, indicating impairment of glycosphingolipid (GSL) metabolism in BBS. Consequently, we tested whether BBS pathology in Bbs2^−/− mice can be reversed by targeting the underlying ciliary defect via reduction of GSL metabolism. Inhibition of GSL synthesis with the glucosylceramide synthase inhibitor Genz-667161 decreases the obesity, liver disease, retinal degeneration and olfaction defect in Bbs2^−/− mice. These effects are secondary to preservation of ciliary structure and signaling, and stimulation of cellular differentiation. In conclusion, reduction of GSL metabolism resolves the multi-organ pathology of Bbs2^−/− mice by directly preserving ciliary structure and function towards a normal phenotype. Since this approach does not rely on the correction of the underlying genetic mutation, it might translate successfully as a treatment for other ciliopathies.

Metabolic Depletion of Sphingolipids Does Not Alter Cell Cycle Progression in Chinese Hamster Ovary Cells

The cell cycle is a sequential multi-step process essential for growth and proliferation of cells comprising multicellular organisms. Although a number of proteins are known to modulate the cell cycle, the role of lipids in regulation of cell cycle is still emerging. In our previous work, we monitored the role of cholesterol in cell cycle progression in CHO-K1 cells. Since sphingolipids enjoy a functionally synergistic relationship with membrane cholesterol, in this work, we explored whether sphingolipids could modulate the eukaryotic cell cycle using CHO-K1 cells. Sphingolipids are essential components of eukaryotic cell membranes and are involved in a number of important cellular functions. To comprehensively monitor the role of sphingolipids on cell cycle progression, we carried out metabolic depletion of sphingolipids in CHO-K1 cells using inhibitors (fumonisin B₁, myriocin, and PDMP) that block specific steps of the sphingolipid biosynthetic pathway and examined their effect on individual cell cycle phases. Our results show that metabolic inhibitors led to significant reduction in specific sphingolipids, yet such inhibition in sphingolipid biosynthesis did not show any effect on cell cycle progression in CHO-K1 cells. We speculate that any role of sphingolipids on cell cycle progression could be context and cell-type dependent, and cancer cells could be a better choice for monitoring such regulation, since sphingolipids are differentially modulated in these cells.

Glucosylceramide and galactosylceramide, small glycosphingolipids with significant impact on health and disease

Numerous clinical observations and exploitation of cellular and animal models indicate that glucosylceramide (GlcCer) and galactosylceramide (GalCer) are involved in many physiological and pathological phenomena. In many cases, the biological importance of these monohexosylcermides has been shown indirectly as the result of studies on enzymes involved in their synthesis and degradation. Under physiological conditions, GalCer plays a key role in the maintenance of proper structure and stability of myelin and differentiation of oligodendrocytes. On the other hand, GlcCer is necessary for the proper functions of epidermis. Such an important lysosomal storage disease as Gaucher disease (GD) and a neurodegenerative disorder as Parkinson’s disease are characterized by mutations in the GBA1 gene, decreased activity of lysosomal GBA1 glucosylceramidase and accumulation of GlcCer. In contrast, another lysosomal disease, Krabbe disease, is associated with mutations in the GALC gene, resulting in deficiency or decreased activity of lysosomal galactosylceramidase and accumulation of GalCer and galactosylsphingosine. Little is known about the role of both monohexosylceramides in tumor progression; however, numerous studies indicate that GlcCer and GalCer play important roles in the development of multidrug-resistance by cancer cells. It was shown that GlcCer is able to provoke immune reaction and acts as a self-antigen in GD. On the other hand, GalCer was recognized as an important cellular receptor for HIV-1. Altogether, these two molecules are excellent examples of how slight differences in chemical composition and molecular conformation contribute to profound differences in their physicochemical properties and biological functions.

Sphingolipids in embryonic development, cell cycle regulation, and stemness - Implications for polyploidy in tumors

The aberrant biology of polyploid giant cancer cells (PGCC) includes dysregulation of the cell cycle, induction of stress responses, and dedifferentiation, all of which are likely accompanied by adaptations in biophysical properties and metabolic activity. Sphingolipids are the second largest class of membrane lipids and play important roles in many aspects of cell biology that are potentially relevant to polyploidy. We have recently shown that the function of the sphingolipid enzyme acid ceramidase (ASAH1) is critical for the ability of PGCC to generate progeny by depolyploidization but mechanisms by which sphingolipids contribute to polyploidy and generation of offspring with stem-like properties remain elusive. This review discusses the role of sphingolipids during embryonic development, cell cycle regulation, and stem cells in an effort to highlight parallels to polyploidy.

Glycosphingolipid metabolism and polycystic kidney disease

Sphingolipids and glycosphingolipids are classes of structurally and functionally important lipids that regulate multiple cellular processes, including membrane organization, proliferation, cell cycle regulation, apoptosis, transport, migration, and inflammatory signalling pathways. Imbalances in sphingolipid levels or subcellular localization result in dysregulated cellular processes and lead to the development and progression of multiple disorders, including polycystic kidney disease. This review will describe metabolic pathways of glycosphingolipids with a focus on the evidence linking glycosphingolipid mediated regulation of cell signalling, lipid microdomains, cilia, and polycystic kidney disease. We will discuss molecular mechanisms of glycosphingolipid dysregulation and their impact on cystogenesis. We will further highlight how modulation of sphingolipid metabolism can be translated into new approaches for the treatment of polycystic kidney disease and describe current clinical studies with glucosylceramide synthase inhibitors in Autosomal Dominant Polycystic Kidney Disease.

Glucosylceramide synthase silencing combined with the receptor tyrosine kinase inhibitor axitinib as a new multimodal strategy for glioblastoma

A great deal of evidence revealing that lipid metabolism is drastically altered during tumorigenesis has been accumulated. In this work, glucosylceramide synthase (GCS) was targeted, using RNA interference technology (siRNAs), in U87 and DBTRG human glioblastoma (GBM) cells, as in both cell types GCS showed to be overexpressed with respect to normal human astrocytes. The efficacy of a combined therapy to tackle GBM, allying GCS silencing to the new generation chemotherapeutics sunitinib and axitinib, or to the alkylating drugs etoposide and temozolomide, is evaluated here for the first time. With this purpose, studies addressing GBM cell viability and proliferation, cell cycle and apoptosis were performed, which revealed that combination of GCS silencing with axitinib treatment represents a promising therapeutic approach. The reduction of cell viability induced by this combined therapy is proposed to be mediated by excessive production of reactive oxygen species. This work, identifying GCS as a key molecular target to increase GBM susceptibility to a new generation chemotherapeutic, opens windows to the development of innovative strategies to halt GBM recurrence after surgical resection.

Multiple Roles of Glycans in Hematological Malignancies

The three types of blood cells (red blood cells for carrying oxygen, white blood cells for immune protection, and platelets for wound clotting) arise from hematopoietic stem/progenitor cells in the adult bone marrow, and function in physiological regulation and communication with local microenvironments to maintain systemic homeostasis. Hematological malignancies are relatively uncommon malignant disorders derived from the two major blood cell lineages: myeloid (leukemia) and lymphoid (lymphoma). Malignant clones lose their regulatory mechanisms, resulting in production of a large number of dysfunctional cells and destruction of normal hematopoiesis. Glycans are one of the four major types of essential biological macromolecules, along with nucleic acids, proteins, and lipids. Major glycan subgroups are N-glycans, O-glycans, glycosaminoglycans, and glycosphingolipids. Aberrant expression of glycan structures, resulting from dysregulation of glycan-related genes, is associated with cancer development and progression in terms of cell signaling and communication, tumor cell dissociation and invasion, cell-matrix interactions, tumor angiogenesis, immune modulation, and metastasis formation. Aberrant glycan expression occurs in most hematological malignancies, notably acute myeloid leukemia, myeloproliferative neoplasms, and multiple myeloma, etc. Here, we review recent research advances regarding aberrant glycans, their related genes, and their roles in hematological malignancies. Our improved understanding of the mechanisms that underlie aberrant patterns of glycosylation will lead to development of novel, more effective therapeutic approaches targeted to hematological malignancies.

Lysosomal phospholipase A2

Lysosomal phospholipase A2 (PLA2G15) is a ubiquitous enzyme uniquely characterized by a subcellular localization to the lysosome and late endosome. PLA2G15 has an acidic pH optimum, is calcium independent, and acts as a transacylase in the presence of N-acetyl-sphingosine as an acceptor. Recent studies aided by the delineation of the crystal structure of PLA2G15 have clarified further the catalytic mechanism, sn-1 versus sn-2 specificity, and the basis whereby cationic amphiphilic drugs inhibit its activity. PLA2G15 has recently been shown to hydrolyze short chain oxidized phospholipids which access the catalytic site directly based on their aqueous solubility. Studies on the PLA2G15 null mouse suggest a role for the enzyme in the catabolism of pulmonary surfactant. PLA2G15 may also have a role in host defense and in the processing of lipid antigens for presentation by CD1 proteins. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

Sphingolipid abnormalities in cancer multidrug resistance: Chicken or egg?

The cancer multidrug resistance (MDR) phenotype encompasses a myriad of molecular, genetic and cellular alterations resulting from progressive oncogenic transformation and selection. Drug efflux transporters, in particular the MDR P-glycoprotein ABCB1, play an important role in MDR but cannot confer the complete phenotype alone indicating parallel alterations are prerequisite. Sphingolipids are essential constituents of lipid raft domains and directly participate in functionalization of transmembrane proteins, including providing an optimal lipid microenvironment for multidrug transporters, and are also perturbed in cancer. Here we postulate that increased sphingomyelin content, developing early in some cancers, recruits and functionalizes plasma membrane ABCB1 conferring a state of partial MDR, which is completed by glycosphingolipid disturbance and the appearance of intracellular vesicular ABCB1. In this review, the independent and interdependent roles of sphingolipid alterations and ABCB1 upregulation during the transformation process and resultant conferment of partial and complete MDR phenotypes are discussed.

Tumor suppressive functions of ceramide: evidence and mechanisms

Studies over the past two decades have identified ceramide as a multifunctional central molecule in the sphingolipid biosynthetic pathway. Given its diverse tumor suppressive activities, molecular understanding of ceramide action will produce fundamental insights into processes that limit tumorigenesis and may identify key molecular targets for therapeutic intervention. Ceramide can be activated by a diverse array of stresses such as heat shock, genotoxic damage, oxidative stress and anticancer drugs. Ceramide triggers a variety of tumor suppressive and anti-proliferative cellular programs such as apoptosis, autophagy, senescence, and necroptosis by activating or repressing key effector molecules. Defects in ceramide generation and metabolism in cancer contribute to tumor cell survival and resistance to chemotherapy. The potent and versatile anticancer activity profile of ceramide has motivated drug development efforts to (re-)activate ceramide in established tumors. This review focuses on our current understanding of the tumor suppressive functions of ceramide and highlights the potential downstream targets of ceramide which are involved in its tumor suppressive action.

Regulation of Sphingolipid Biosynthesis by the Morphogenesis Checkpoint Kinase Swe1

Sphingolipid (SL) biosynthesis is negatively regulated by the highly conserved endoplasmic reticulum-localized Orm family proteins. Defective SL synthesis in Saccharomyces cerevisiae leads to increased phosphorylation and inhibition of Orm proteins by the kinase Ypk1. Here we present evidence that the yeast morphogenesis checkpoint kinase, Swe1, regulates SL biosynthesis independent of the Ypk1 pathway. Deletion of the Swe1 kinase renders mutant cells sensitive to serine palmitoyltransferase inhibition due to impaired sphingoid long-chain base synthesis. Based on these data and previous results, we suggest that Swe1 kinase perceives alterations in SL homeostasis, activates SL synthesis, and may thus represent the missing regulatory link that controls the SL rheostat during the cell cycle.

Tamoxifen augments the innate immune function of neutrophils through modulation of intracellular ceramide

Tamoxifen is a selective estrogen receptor modulator widely used for the treatment of breast cancer. In addition to its activity as an estrogen receptor agonist/antagonist, tamoxifen also modulates sphingolipid biosynthesis, which has been shown to play an important role in the regulation of neutrophil activity. Here, we find that tamoxifen stimulation enhances several pro-inflammatory pathways in human neutrophils, including chemotaxis, phagocytosis and neutrophil extracellular trap (NET) formation. The enhancement of NET production occurs via a ceramide/PKCζ-mediated pathway, and treatment with synthetic ceramide is sufficient to promote NET formation. Pretreatment of human neutrophils with tamoxifen boosts neutrophil bactericidal capacity against a variety of pathogens in vitro and enhances clearance of the leading human pathogen methicillin-resistant Staphylococcus aureus in vivo. Our results suggest that tamoxifen, and the lipid signaling pathways it modulates, merit further exploration as targets for boosting host innate immune function.

The ceramide kinase inhibitor NVP-231 inhibits breast and lung cancer cell proliferation by inducing M phase arrest and subsequent cell death

BACKGROUND AND PURPOSE

Ceramide kinase (CerK) catalyzes the generation of ceramide-1-phosphate which may regulate various cellular functions, including inflammatory reactions and cell growth. Here, we studied the effect of a recently developed CerK inhibitor, NVP-231, on cancer cell proliferation and viability and investigated the role of cell cycle regulators implicated in these responses.

EXPERIMENTAL APPROACH

The breast and lung cancer cell lines MCF-7 and NCI-H358 were treated with increasing concentrations of NVP-231 and DNA synthesis, colony formation and cell death were determined. Flow cytometry was performed to analyse cell cycle distribution of cells and Western blot analysis was used to detect changes in cell cycle regulator expression and activation.

KEY RESULTS

In both cell lines, NVP-231 concentration-dependently reduced cell viability, DNA synthesis and colony formation. Moreover it induced apoptosis, as measured by increased DNA fragmentation and caspase-3 and caspase-9 cleavage. Cell cycle analysis revealed that NVP-231 decreased the number of cells in S phase and induced M phase arrest with an increased mitotic index, as determined by increased histone H3 phosphorylation. The effect on the cell cycle was even more pronounced when NVP-231 treatment was combined with staurosporine. Finally, overexpression of CerK protected, whereas down-regulation of CerK with siRNA sensitized, cells for staurosporine-induced apoptosis.

CONCLUSIONS AND IMPLICATIONS

Our data demonstrate for the first time a crucial role for CerK in the M phase control in cancer cells and suggest its targeted inhibition, using drugs such as NVP-231, in combination with conventional pro-apoptotic chemotherapy.

Eliglustat tartrate, a prototypic glucosylceramide synthase inhibitor

Eliglustat tartrate is a highly specific inhibitor of glucosylceramide synthase, developed for the treatment glucosylceramide-based glycosphingolipidoses. Eliglustat is in late clinical development for Gaucher disease type 1. Phase II and III clinical trials have demonstrated clinical efficacy for eliglustat as a stand-alone agent for newly diagnosed patients that are naïve to prior therapy and for patients who have been previously treated with enzyme replacement therapy. Importantly, the reported toxicity of eliglustat has been limited. Eliglustat will be submitted for the US FDA and EMA review in late 2013. Several structurally unrelated glucosylceramide synthase inhibitors have been identified and are in various stages of development, some of which cross the blood-brain barrier. Targeting glucosylceramide synthesis is also a promising approach for the treatment of type 2 diabetes mellitus, autosomal dominant polycystic kidney disease and certain cancers.

Use of a glycolipid inhibitor to ameliorate renal cancer in a mouse model

In a xenograft model wherein, live renal cancer cells were implanted under the kidney capsule in mice, revealed a 30-fold increase in tumor volume over a period of 26 days and this was accompanied with a 32-fold increase in the level of lactosylceramide (LacCer). Mice fed D- threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of glucosylceramide synthase and lactosylceramide synthase (LCS: β-1,4-GalT-V), showed marked reduction in tumor volume. This was accompanied by a decrease in the mass of lactosylceramide and an increase in glucosylceramide (GlcCer) level. Mechanistic studies revealed that D-PDMP inhibited cell proliferation and angiogenesis by inhibiting p44MAPK, p-AKT-1 pathway and mammalian target for rapamycin (mTOR). By linking glycosphingolipid synthesis with tumor growth, renal cancer progression and regression can be evaluated. Thus inhibiting glycosphingolipid synthesis can be a bonafide target to prevent the progression of other types of cancer.

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.

Inhibition of ceramide metabolism sensitizes human leukemia cells to inhibition of BCL2-like proteins

The identification of novel combinations of effective cancer drugs is required for the successful treatment of cancer patients for a number of reasons. First, many “cancer specific” therapeutics display detrimental patient side-effects and second, there are almost no examples of single agent therapeutics that lead to cures. One strategy to decrease both the effective dose of individual drugs and the potential for therapeutic resistance is to combine drugs that regulate independent pathways that converge on cell death. BCL2-like family members are key proteins that regulate apoptosis. We conducted a screen to identify drugs that could be combined with an inhibitor of anti-apoptotic BCL2-like proteins, ABT-263, to kill human leukemia cells lines. We found that the combination of D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) hydrochloride, an inhibitor of glucosylceramide synthase, potently synergized with ABT-263 in the killing of multiple human leukemia cell lines. Treatment of cells with PDMP and ABT-263 led to dramatic elevation of two pro-apoptotic sphingolipids, namely ceramide and sphingosine. Furthermore, treatment of cells with the sphingosine kinase inhibitor, SKi-II, also dramatically synergized with ABT-263 to kill leukemia cells and similarly increased ceramides and sphingosine. Data suggest that synergism with ABT-263 requires accumulation of ceramides and sphingosine, as AMP-deoxynojirimycin, (an inhibitor of the glycosphingolipid pathway) did not elevate ceramides or sphingosine and importantly did not sensitize cells to ABT-263 treatment. Taken together, our data suggest that combining inhibitors of anti-apoptotic BCL2-like proteins with drugs that alter the balance of bioactive sphingolipids will be a powerful combination for the treatment of human cancers.

B-cell receptor triggers drug sensitivity of primary CLL cells by controlling glucosylation of ceramides

Survival of chronic lymphocytic leukemia (CLL) cells is triggered by several stimuli, such as the B-cell receptor (BCR), CD40 ligand (CD40L), or interleukin-4 (IL-4). We identified that these stimuli regulate apoptosis resistance by modulating sphingolipid metabolism. Applying liquid chromatography electrospray ionization tandem mass spectrometry, we revealed a significant decrease of proapoptotic ceramide in BCR/IL-4/CD40L–stimulated primary CLL cells compared with untreated controls. Antiapoptotic glucosylceramide levels were significantly increased after BCR cross-linking. We identified BCR engagement to catalyze the crucial modification of ceramide to glucosylceramide via UDP-glucose ceramide glucosyltransferase (UGCG). Besides specific UGCG inhibitors, our data demonstrate that IgM-mediated UGCG expression was inhibited by the novel and highly effective PI3Kδ and BTK inhibitors CAL-101 and PCI-32765, which reverted IgM-induced resistance toward apoptosis of CLL cells. Sphingolipids were recently shown to be crucial for mediation of apoptosis via mitochondria. Our data reveal ABT-737, a mitochondria-targeting drug, as interesting candidate partner for PI3Kδ and BTK inhibition, resulting in synergistic apoptosis, even under protection by the BCR. In summary, we identified the mode of action of novel kinase inhibitors CAL-101 and PCI-32765 by controlling the UGCG-mediated ceramide/glucosylceramide equilibrium as a downstream molecular switch of BCR signaling, also providing novel targeted treatment options beyond current chemotherapy-based regimens.

Age-dependent changes in the sphingolipid composition of mouse CD4+ T cell membranes and immune synapses implicate glucosylceramides in age-related T cell dysfunction

To determine whether changes in sphingolipid composition are associated with age-related immune dysfunction, we analyzed the core sphingolipidome (i.e., all of the metabolites through the first headgroup additions) of young and aged CD4⁺ T cells. Since sphingolipids influence the biophysical properties of membranes, we evaluated the compositions of immune synapse (IS) and non-IS fractions prepared by magnetic immuno-isolation. Broadly, increased amounts of sphingomyelins, dihydrosphingomyelins and ceramides were found in aged CD4⁺ T cells. After normalizing for total sphingolipid content, a statistically significant decrease in the molar fraction of glucosylceramides was evident in both the non-IS and IS fractions of aged T cells. This change was balanced by less dramatic increases in the molar fractions of sphingomyelins and dihydrosphingomyelins in aged CD4⁺ T cells. In vitro, the direct or enzymatic enhancement of ceramide levels decreased CD4⁺ T cell proliferation without regard for the age of the responding T cells. In contrast, the in vitro inhibition of glucosylceramidase preferentially increased the proliferation of aged CD4⁺ T cells. These results suggest that reductions in glucosylceramide abundance contribute to age-related impairments in CD4⁺ T cell function.

Reversion of P-glycoprotein-mediated multidrug resistance by guggulsterone in multidrug-resistant human cancer cell lines

Multidrug resistance (MDR) presents a serious problem in cancer chemotherapy. Our previous studies have shown that guggulsterone could reverse MDR through inhibiting the function and expression of P-glycoprotein (P-gp). The present study is to further investigate the reversal effects of guggulsterone on MDR in drug-resistant cancer cell lines. The effects of guggulsterone on MDR1mRNA gene expression, intracellular pH, P-gp ATPase activity and glucosylceramide synthase (GCS) expression were assessed by RT-PCR, Laser Scanning Confocal Microscope using the pH-sensitive fluorescent probe BCECF-AM, Pgp-Glo assay system, and flow cytometric technology, respectively. The results showed that guggulsterone ranging from 2.5 to 80 μM significantly promoted the activity of P-gp ATPase in a dose-dependent manner. The intracellular pH of K562/DOX cells was found to be higher than K562 cells. After treatment with guggulsterone (1, 3, 10, 30, 100 μM), intracellular pH of K562/DOX cells decreased in a dose- and time-dependent manner. However, the present study revealed that guggulsterone ranging from 3 to 100 μM had little influence on MDR1 gene expression in K562/DOX cells. Further, the isogenic doxorubicin-resistant MCF-7/DOX cells exhibited a 4.9-fold increase in GCS level as compared with parental MCF-7 human breast cancer cells. After treatment with guggulsterone (0.1, 1, 10 μM) for 48 h, MCF-7/DOX cells were found to have no change of GCS protein expression amount. Guggulsterone might be a potent MDR reversal agent, and its mechanism on MDR needs more research.

Loss of GM3 synthase gene, but not sphingosine kinase 1, is protective against murine nephronophthisis-related polycystic kidney disease

Genetic forms of polycystic kidney diseases (PKDs), including nephronophthisis, are characterized by formation of fluid-filled cysts in the kidneys and progression to end-stage renal disease. No therapies are currently available to treat cystic diseases, making it imperative to dissect molecular mechanisms in search of therapeutic targets. Accumulating evidence suggests a pathogenic role for glucosylceramide (GlcCer) in multiple forms of PKD. It is not known, however, whether other structural glycosphingolipids (GSLs) or bioactive signaling sphingolipids (SLs) modulate cystogenesis. Therefore, we set out to address the role of a specific GSL (ganglioside GM3) and signaling SL (sphingosine-1-phosphate, S1P) in PKD progression, using the jck mouse model of nephronopthisis. To define the role of GM3 accumulation in cystogenesis, we crossed jck mice with mice carrying a targeted mutation in the GM3 synthase (St3gal5) gene. GM3-deficient jck mice displayed milder PKD, revealing a pivotal role for ganglioside GM3. Mechanistic changes in regulation of the cell-cycle machinery and Akt-mTOR signaling were consistent with reduced cystogenesis. Dramatic overexpression of sphingosine kinase 1 (Sphk1) mRNA in jck kidneys suggested a pathogenic role for S1P. Surprisingly, genetic loss of Sphk1 exacerbated cystogenesis and was associated with increased levels of GlcCer and GM3. On the other hand, increasing S1P accumulation through pharmacologic inhibition of S1P lyase had no effect on the progression of cystogenesis or kidney GSL levels. Together, these data suggest that genes involved in the SL metabolism may be modifiers of cystogenesis, and suggest GM3 synthase as a new anti-cystic therapeutic target.

Neurofibromatosis-like phenotype in Drosophila caused by lack of glucosylceramide extension

Glycosphingolipids (GSLs) are of fundamental importance in the nervous system. However, the molecular details associated with GSL function are largely unknown, in part because of the complexity of GSL biosynthesis in vertebrates. In Drosophila, only one major GSL biosynthetic pathway exists, controlled by the glycosyltransferase Egghead (Egh). Here we discovered that loss of Egh causes overgrowth of peripheral nerves and attraction of immune cells to the nerves. This phenotype is reminiscent of the human disorder neurofibromatosis type 1, which is characterized by disfiguring nerve sheath tumors with mast cell infiltration, increased cancer risk, and learning deficits. Neurofibromatosis type 1 is due to a reduction of the tumor suppressor neurofibromin, a negative regulator of the small GTPase Ras. Enhanced Ras signaling promotes glial growth through activation of phosphatidylinositol 3-kinase (PI3K) and its downstream kinase Akt. We find that overgrowth of peripheral nerves in egh mutants is suppressed by down-regulation of the PI3K signaling pathway by expression of either dominant-negative PI3K, the tumor suppressor PTEN, or the transcription factor FOXO in the subperineurial glia. These results show that loss of the glycosyltransferase Egh affects membrane signaling and activation of PI3K signaling in glia of the peripheral nervous system, and suggest that glycosyltransferases may suppress proliferation.

Murine noroviruses bind glycolipid and glycoprotein attachment receptors in a strain-dependent manner

Human norovirus infections are the most common cause of acute nonbacterial gastroenteritis in humans worldwide, and glycan binding plays an important role in the susceptibility to these infections. However, due to the lack of an efficient cell culture system or small animal model for human noroviruses, little is known about the biological role of glycan binding during infection. Murine noroviruses (MNV) are also enteric viruses that bind to cell surface glycans, but in contrast to their human counterparts, they can be grown in tissue culture and a small animal host. In this study, we determined glycan-binding specificities of the MNV strains MNV-1 and CR3 in vitro, identified molecular determinants of glycan binding, and analyzed infection in vivo. We showed that unlike MNV-1, CR3 binding to murine macrophages was resistant to neuraminidase treatment and glycosphingolipid depletion. Both strains depended on N-linked glycoproteins for binding, while only MNV-1 attachment to macrophages was sensitive to O-linked glycoprotein depletion. In vivo, CR3 showed differences in tissue tropism compared to MNV-1 by replicating in the large intestine. Mapping of a glycan-binding site in the MNV-1 capsid by reverse genetics identified a region topologically similar to the histo-blood group antigen (HBGA)-binding sites of the human norovirus strain VA387. The recombinant virus showed distinct changes in tissue tropism compared to wild-type virus. Taken together, our data demonstrate that MNV strains evolved multiple strategies to bind different glycan receptors on the surface of murine macrophages and that glycan binding contributes to tissue tropism in vivo.

Glucosylceramide synthase inhibitor PDMP sensitizes chronic myeloid leukemia T315I mutant to Bcr-Abl inhibitor and cooperatively induces glycogen synthase kinase-3-regulated apoptosis

Inactivation of glycogen synthase kinase (GSK)-3 has been implicated in cancer progression. Previously, we showed an abundance of inactive GSK-3 in the human chronic myeloid leukemia (CML) cell line. CML is a hematopoietic malignancy caused by an oncogenic Bcr-Abl tyrosine kinase. In Bcr-Abl signaling, the role of GSK-3 is not well defined. Here, we report that enforced expression of constitutively active GSK-3 reduced proliferation and increased Bcr-Abl inhibition-induced apoptosis by nearly 1-fold. Bcr-Abl inhibition activated GSK-3 and GSK-3-dependent apoptosis. Inactivation of GSK-3 by Bcr-Abl activity is, therefore, confirmed. To reactivate GSK-3, we used glucosylceramide synthase (GCS) inhibitor PDMP to accumulate endogenous ceramide, a tumor-suppressor sphingolipid and a potent GSK-3 activator. We found that either PDMP or silence of GCS increased Bcr-Abl inhibition-induced GSK-3 activation and apoptosis. Furthermore, PDMP sensitized the most clinical problematic drug-resistant CML T315I mutant to Bcr-Abl inhibitor GNF-2-, imatinib-, or nilotinib-induced apoptosis by >5-fold. Combining PDMP and GNF-2 eliminated transplanted-CML-T315I-mutants in vivo and dose dependently sensitized primary cells from CML T315I patients to GNF-2-induced proliferation inhibition and apoptosis. The synergistic efficacy was Bcr-Abl restricted and correlated to increased intracellular ceramide levels and acted through GSK-3-mediated apoptosis. This study suggests a feasible novel anti-CML strategy by accumulating endogenous ceramide to reactivate GSK-3 and abrogate drug resistance.

Inhibition of ganglioside synthesis reduces the neuronal survival activity of astrocytes

Gangliosides are sialic acid-containing sphingolipids that have promoting effects on the survival, growth, and functions of neurons in the central nervous system. Glial cells including astrocytes assist neurons with regard to their proliferation, maintenance, and function. In the present study, we found that astrocytes with ganglioside depletion caused by treatment with d-threo-1-phenyl-1-2-decanoylamino-3-morpholino-1-propanol (d-PDMP) exhibited reduced neuronal survival activity, as evaluated using low-density hippocampal cultures. The reduction was rescued by a ganglioside, GM3, suggesting that GM3 has significant effects on the glial cell activities that help neurons to proliferate and be maintained.

Group XV phospholipase A₂, a lysosomal phospholipase A₂

A phospholipase A₂ was identified from MDCK cell homogenates with broad specificity toward glycerophospholipids including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylglycerol. The phospholipase has the unique ability to transacylate short chain ceramides. This phospholipase is calcium-independent, localized to lysosomes, and has an acidic pH optimum. The enzyme was purified from bovine brain and found to be a water-soluble glycoprotein consisting of a single peptide chain with a molecular weight of 45 kDa. The primary structure deduced from the DNA sequences is highly conserved between chordates. The enzyme was named lysosomal phospholipase A₂ (LPLA₂) and subsequently designated group XV phospholipase A₂. LPLA₂ has 49% of amino acid sequence identity to lecithin-cholesterol acyltransferase and is a member of the αβ-hydrolase superfamily. LPLA₂ is highly expressed in alveolar macrophages. A marked accumulation of glycerophospholipids and extensive lamellar inclusion bodies, a hallmark of cellular phospholipidosis, is observed in alveolar macrophages in LPLA₂(-/-) mice. This defect can also be reproduced in macrophages that are exposed to cationic amphiphilic drugs such as amiodarone. In addition, older LPLA₂(-/-) mice develop a phenotype similar to human autoimmune disease. These observations indicate that LPLA₂ may play a primary role in phospholipid homeostasis, drug toxicity, and host defense.

Human glycolipid transfer protein gene (GLTP) expression is regulated by Sp1 and Sp3: involvement of the bioactive sphingolipid ceramide

Glycolipid transfer protein (GLTP) accelerates glycolipid intermembrane transfer via a unique lipid transfer/binding fold (GLTP fold) that defines the GLTP superfamily and is the prototype for functional GLTP-like domains in larger proteins, i.e. FAPP2. Human GLTP is encoded by the single-copy GLTP gene on chromosome 12 (12q24.11 locus), but regulation of GLTP gene expression remains completely unexplored. Herein, the ability of glycosphingolipids (and their sphingolipid metabolites) to regulate the transcriptional expression of GLTP via its promoter has been evaluated. Using luciferase and GFP reporters in concert with deletion mutants, the constitutive and basal (225 bp; ∼78% G+C) human GLTP promoters have been defined along with adjacent regulatory elements. Despite high G+C content, translational regulation was not evident by the mammalian target of rapamycin pathway. Four GC-boxes were shown to be functional Sp1/Sp3 transcription factor binding sites. Mutation of one GC-box was particularly detrimental to GLTP transcriptional activity. Sp1/Sp3 RNA silencing and mithramycin A treatment significantly inhibited GLTP promoter activity. Among tested sphingolipid analogs of glucosylceramide, sulfatide, ganglioside GM1, ceramide 1-phosphate, sphingosine 1-phosphate, dihydroceramide, sphingosine, only ceramide, a nonglycosylated precursor metabolite unable to bind to GLTP protein, induced GLTP promoter activity and raised transcript levels in vivo. Ceramide treatment partially blocked promoter activity decreases induced by Sp1/Sp3 knockdown. Ceramide treatment also altered the in vivo binding affinity of Sp1 and Sp3 for the GLTP promoter and decreased Sp3 acetylation. This study represents the first characterization of any Gltp gene promoter and links human GLTP expression to sphingolipid homeostasis through ceramide.

Inhibition of glucosylceramide accumulation results in effective blockade of polycystic kidney disease in mouse models

Polycystic kidney disease (PKD) represents a family of genetic disorders characterized by renal cystic growth and progression to kidney failure. No treatment is currently available for people with PKD, although possible therapeutic interventions are emerging. Despite genetic and clinical heterogeneity, PKDs have in common defects of cystic epithelia, including increased proliferation, apoptosis and activation of growth regulatory pathways. Sphingolipids and glycosphingolipids are emerging as major regulators of these cellular processes. We sought to evaluate the therapeutic potential for glycosphingolipid modulation as a new approach to treat PKD. Here we demonstrate that kidney glucosylceramide (GlcCer) and ganglioside GM3 levels are higher in human and mouse PKD tissue as compared to normal tissue, regardless of the causative mutation. Blockade of GlcCer accumulation with the GlcCer synthase inhibitor Genz-123346 effectively inhibits cystogenesis in mouse models orthologous to human autosomal dominant PKD (Pkd1 conditional knockout mice) and nephronophthisis (jck and pcy mice). Molecular analysis in vitro and in vivo indicates that Genz-123346 acts through inhibition of the two key pathways dysregulated in PKD: Akt protein kinase-mammalian target of rapamycin signaling and cell cycle machinery. Taken together, our data suggest that inhibition of GlcCer synthesis represents a new and effective treatment option for PKD.

Glucosylceramide synthesis inhibition affects cell cycle progression, membrane trafficking, and stage differentiation in Giardia lamblia

Synthesis of glucosylceramide via glucosylceramide synthase (GCS) is a crucial event in higher eukaryotes, both for the production of complex glycosphingolipids and for regulating cellular levels of ceramide, a potent antiproliferative second messenger. In this study, we explored the dependence of the early branching eukaryote Giardia lamblia on GCS activity. Biochemical analyses revealed that the parasite has a GCS located in endoplasmic reticulum (ER) membranes that is active in proliferating and encysting trophozoites. Pharmacological inhibition of GCS induced aberrant cell division, characterized by arrest of cytokinesis, incomplete cleavage furrow formation, and consequent block of replication. Importantly, we showed that increased ceramide levels were responsible for the cytokinesis arrest. In addition, GCS inhibition resulted in prominent ultrastructural abnormalities, including accumulation of cytosolic vesicles, enlarged lysosomes, and clathrin disorganization. Moreover, anterograde trafficking of the encystations-specific protein CWP1 was severely compromised and resulted in inhibition of stage differentiation. Our results reveal novel aspects of lipid metabolism in G. lamblia and specifically highlight the vital role of GCS in regulating cell cycle progression, membrane trafficking events, and stage differentiation in this parasite. In addition, we identified ceramide as a potent bioactive molecule, underscoring the universal conservation of ceramide signaling in eukaryotes.

Ganglioside synthase knockout in oncogene-transformed fibroblasts depletes gangliosides and impairs tumor growth

Biologically active membrane gangliosides, expressed and released by many human tumors, are hypothesized to significantly impact tumor progression. Lack of a model of complete and specific tumor ganglioside depletion in vivo, however, has hampered elucidation of their role. Here we report the creation of a novel, stable, genetically induced tumor cell system resulting in specific and complete blockade of ganglioside synthesis. Wild type (WT) and GM3 synthase/GM2 synthase double knockout (DKO) murine embryonic fibroblasts were transformed using amphotropic retrovirus-transduced oncogenes (pBABE-c-Myc^T58A+H-Ras^G12V). The transformed cells, WT_t and DKO_t respectively, evidenced comparable integrated copy numbers and oncogene expression. Ganglioside synthesis was completely blocked in the DKO_t cells, importantly without triggering an alternate pathway of ganglioside synthesis. Ganglioside depletion (to <0.5 nmol/10⁷ cells from 9-11 nmol/10⁷ WT_t or untransfected normal fibroblasts) did not adversely affect cell proliferation kinetics but did reduce cell migration on fibronectin-coated wells, consistent with our previous observations in ganglioside-depleted normal human fibroblasts. Strikingly, despite similar oncogene expression and growth kinetics, DKO_t cells evidenced significantly impaired tumor growth in syngeneic immunocompetent mice, underscoring the pivotal role of tumor cell gangliosides and providing an ideal system for probing their mechanisms of action in vivo.

Hypoxia-induced neuronal apoptosis is mediated by de novo synthesis of ceramide through activation of serine palmitoyltransferase

Cellular hypoxia can lead to cell death or adaptation and has important effects on development, physiology, and pathology. Here, we investigated the role and regulation of ceramide in hypoxia-induced apoptosis of SH-SY5Y neuroblastoma cells. Hypoxia increased the ceramide concentration; subsequently, we observed biochemical changes indicative of apoptosis, such as DNA fragmentation, nuclear staining, and poly ADP-ribose polymerase (PARP) cleavage. The hypoxic cell death was potently inhibited by a caspase inhibitor, zVAD-fmk (benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone). l-Cycloserine, a serine palmitoyltransferase (SPT) inhibitor, and fumonisin B(1) (FB(1)), a ceramide synthase inhibitor, inhibited the hypoxia-induced increase in ceramide, indicating that the increase occurred via the de novo pathway. Hypoxia increased the activity and protein levels of SPT2, suggesting that the hypoxia-induced increase in ceramide is due to the transcriptional up-regulation of SPT2. Specific siRNA of SPT2 prevented hypoxia-induced cell death and ceramide production. However, hypoxia also increased the cellular level of glucosylceramide, which was inhibited by a glucosylceramide synthase (GCS) inhibitor and specific siRNA, but not a ceramidase inhibitor. The increase in glucosylceramide was accompanied by increases in both PARP cleavage and DNA fragmentation. Together, the current results suggest that both SPT and GCS may regulate the cellular level of ceramide, and thus may be critical enzymes for deciding the fate of the cells exposed to hypoxia.

Downregulation of neutral ceramidase by gemcitabine: Implications for cell cycle regulation

Gemcitabine (GMZ) is a chemotherapeutic agent with well established effects on cell growth arrest and apoptosis. In this study, we investigated the potential roles of bioactive sphingolipids in mediating the growth suppressing effects of GMZ on a polyoma middle T transformed murine endothelial cell line. After 12-hour GMZ (0.6 microM) treatment, cell growth was arrested at the G(0)/G(1) phase as detected by flow cytometric cell cycle analysis and MTT cell viability analysis, and this was accompanied by dephosphorylation of the retinoblastoma protein (Rb). Furthermore, GMZ treatment resulted in increased levels of specifically the very long chain ceramides as determined by mass spectrometry. Mechanistically, GMZ did not appear to affect the activities of many enzymes of ceramide metabolism; however, GMZ caused a selective reduction in the protein levels of neutral ceramidase (NCDase), as indicated by Western blot analysis, with a concomitant decrease in NCDase activity. The significance of NCDase loss on cell cycle regulation was investigated by specific knockdown of the enzyme using small interfering RNA (siRNA). Interestingly, NCDase siRNA transfection was sufficient to induce a cell cycle arrest at G(0)/G(1) and an increase in total ceramide levels, with significant elevation in very long chain ceramides (C(24:1) and C(24:0)). NCDase siRNA also induced Rb dephosphorylation. These data provide evidence for a novel mechanism of action for GMZ and highlight downregulation of NCDase as a critical step in GMZ-mediated ceramide elevation and cell cycle arrest.

Ganglioside-linked terminal sialic acid moieties on murine macrophages function as attachment receptors for murine noroviruses

Noroviruses are the major cause of nonbacterial gastroenteritis in humans. However, little is known regarding the norovirus life cycle, including cell binding and entry. In contrast to human noroviruses, the recently discovered murine norovirus 1 (MNV-1) readily infects murine macrophages and dendritic cells in culture. Many viruses, including the related feline calicivirus, use terminal sialic acids (SA) as receptors for infection. Therefore, we tested whether SA moieties play a role during MNV-1 infection of murine macrophages. Competition with SA-binding lectins and neuraminidase treatment led to a reduction in MNV-1 binding and infection in cultured and primary murine macrophages, suggesting a role for SA during the initial steps of the MNV-1 life cycle. Because SA moieties can be attached to glycolipids (i.e., gangliosides), we next determined whether MNV-1 uses gangliosides during infection. The gangliosides GD1a, GM1, and asialo-GM1 (GA1) are natural components of murine macrophages. MNV-1 bound to ganglioside GD1a, which is characterized by an SA on the terminal galactose, but not to GM1 or asialo-GM1 in an enzyme-linked immunosorbent assay. The depletion of gangliosides using an inhibitor of glycosylceramide synthase (d-threo-P4) led to a reduction of MNV-1 binding and infection in cultured and primary murine macrophages. This defect was specifically rescued by the addition of GD1a. A similar phenotype was observed for MNV field strains WU11 (GV/WU11/2005/USA) and S99 (GV/Berlin/2006/DE). In conclusion, our data indicate that MNV can use terminal SA on gangliosides as attachment receptors during binding to murine macrophages.

VEGF recruits lactosylceramide to induce endothelial cell adhesion molecule expression and angiogenesis in vitro and in vivo

Angiogenesis is largely driven by vascular endothelial growth factor (VEGF). However, the role of lipid second messengers such as lactosylceramide (LacCer) and LacCer synthase in angiogenesis is not well understood. We have determined the distribution of various LacCer synthase mRNA transcripts using sequential analysis of gene expression (SAGE). Endothelial cells from colon cancer tissues had a 4.5-fold increase in a LacCer synthase transcript (beta1,4GalT-V) as compared to normal colon tissue endothelial cells. Consequently, our focus turned to understanding the role of this enzyme in regulating VEGF-induced angiogenesis in vitro and in vivo. Herein, we show that in human endothelial cells, VEGF-induced angiogenesis is mitigated by dimethylsphingosine and suramin; inhibitors of sphingosine kinase 1(SphK-1) and sphingosine1-phosphate receptor 1(S1P (1)), respectively, and this were bypassed by LacCer but not by S1P. VEGF and basic fibroblast growth factor-induced angiogenesis was mitigated by PDMP; an inhibitor of glucosylceramide synthase and LacCer synthase in human umbilical vein endothelial cells (HUVEC) and human aortic endothelial cells (HAEC). Likewise, GalT-V gene ablation using corresponding siRNA also mitigated VEGF-induced angiogenesis. In Matrigel plug angiogenesis assay in nude mice, angiogenesis was markedly inhibited by D-PDMP with concordantly diminished LacCer synthase activity. Mechanistic studies revealed that the use of LY294002, a PI3 kinase inhibitor, mitigated VEGF-induced expression of platelet-endothelial cell adhesion molecule (PECAM-1/CD31); the trans-endothelial migration of a monocyte cell line (U-937) and angiogenesis in HAEC cells. Since this enzyme is a target for VEGF action and LacCer serves as a lipid second messenger in inducing angiogenesis in vitro and in vivo, novel therapeutic approaches may be developed using our findings to mitigate colon cancer.

Ceramide-induced G2 arrest in rhabdomyosarcoma (RMS) cells requires p21Cip1/Waf1 induction and is prevented by MDM2 overexpression

The sphingoplipid ceramide is responsible for a diverse range of biochemical and cellular responses including a putative role in modulating cell cycle progression. Herein, we describe that an accumulation of ceramide, achieved through the exogenous application of C(6)-ceramide or exposure to sphingomyelinase, induces a G(2) arrest in Rhabdomyosarcoma (RMS) cell lines. Utilizing the RMS cell line RD, we show that this G(2) arrest required the rapid induction of p21(Cip1/Waf1) independent of DNA damage. This was followed at later time points (48 h) by the commitment to apoptosis. Apoptosis was prevented by Bcl-2 overexpression, but permitted the maintenance of elevated p21(Cip1/Waf1) protein expression and the stabilization of the G(2) arrest response. Inhibition of p21(Cip1/Waf1) protein synthesis with cyclohexamide (CHX) or silencing of p21(Cip1/Waf1) with siRNA, prevented ceramide-mediated G(2) arrest and the late induction of apoptosis. Further, adopting the recent discovery that murine double minute 2 (MDM2) controls p21(Cip1/Waf1) expression by presenting this CDK inhibitor to the proteasome for degradation, RD cells overexpressing MDM2 abrogated ceramide-mediated p21(Cip1/Waf1) induction, G(2) arrest and the late ensuing apoptosis. Collectively, these data further support the notion that ceramide accumulation can modulate cell cycle progression. Additionally, these observations highlight MDM2 expression and proteasomal activity as key determinants of the cellular response to ceramide accumulation.

Ganglioside GM2-tetraspanin CD82 complex inhibits met and its cross-talk with integrins, providing a basis for control of cell motility through glycosynapse

Glycosphingolipids (GSLs) at the cell surface membrane are associated or complexed with signal transducers (Src family kinases and small G-proteins), tetraspanins, growth factor receptors, and integrins. Such organizational framework, defining GSL-modulated or -dependent cell adhesion, motility, and growth, is termed "glycosynapse" (Hakomori, S., and Handa, K. (2002) FEBS Lett. 531, 88-92; Hakomori, S. (2004) Ann. Braz. Acad. Sci. 76, 553-572). We describe here the functional organization of the glycosynaptic microdomain, and the mechanisms for control of cell motility and invasiveness, in normal bladder epithelial HCV29 cells versus highly invasive bladder cancer YTS1 cells, both derived from transitional epithelia. (i) Ganglioside GM2, but not GM3 or globoside, interacted specifically with tetraspanin CD82, and such a complex inhibited hepatocyte growth factor (HGF)-induced activation of Met tyrosine kinase in a dose-dependent manner. (ii) Depletion of GM2 in HCV29 cells by treatment with D-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (P4), or reduction of CD82 expression by RNA interference, significantly enhanced HGF-induced Met tyrosine kinase and cell motility. (iii) In contrast, YTS1 cells, lacking CD82, displayed HGF-independent activation of Met tyrosine kinase and high cell motility. Transfection of the CD82 gene to YTS1 inhibited HGF dose-dependent Met tyrosine kinase activity and cell motility, due to formation of the GM2-CD82 complex. (iv) Adhesion of YTS1 or YTS1/CD82 cells to laminin-5-coated plates, as compared with noncoated plates, strongly enhanced Met activation, and the degree of activation was further increased in association with GSL depletion by P4. Laminin-5-dependent Met activation was minimal in HCV29 cells. These findings indicate that GSL, particularly GM2, forms a complex with CD82, and that such complex interacts with Met and thereby inhibits HGF-induced Met tyrosine kinase activity, as well as integrin to Met cross-talk.

Sphingolipids as modulators of cancer cell death: potential therapeutic targets

Through modifications in the fine membrane structure, cell-cell or cell-matrix interactions, and/or modulation of intracellular signaling pathways, sphingolipids can affect the tumorigenic potential of numerous cell types. Whereas ceramide and its metabolites have been described as regulators of cell growth and apoptosis, these lipids as well as other sphingolipid molecules can modulate the ability of malignant cells to grow and resist anticancer treatments, and their susceptibility to non-apoptotic cell deaths. This review summarizes our current knowledge on the properties of sphingolipids in the regulation of cancer cell death and tumor development. It also provides an update on the potential perspectives of manipulating sphingolipid metabolism and using sphingolipid analogues in anticancer therapy.

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.