Accumulation of glucosylceramides in multidrug-resistant cancer cells

Re-configuration of sphingolipid metabolism by oncogenic transformation

The sphingolipids are one of the major lipid families in eukaryotes, incorporating a diverse array of structural variants that exert a powerful influence over cell fate and physiology. Increased expression of sphingosine kinase 1 (SPHK1), which catalyses the synthesis of the pro-survival, pro-angiogenic metabolite sphingosine 1-phosphate (S1P), is well established as a hallmark of multiple cancers. Metabolic alterations that reduce levels of the pro-apoptotic lipid ceramide, particularly its glucosylation by glucosylceramide synthase (GCS), have frequently been associated with cancer drug resistance. However, the simple notion that the balance between ceramide and S1P, often referred to as the sphingolipid rheostat, dictates cell survival contrasts with recent studies showing that highly potent and selective SPHK1 inhibitors do not affect cancer cell proliferation or survival, and studies demonstrating higher ceramide levels in some metastatic cancers. Recent reports have implicated other sphingolipid metabolic enzymes such as acid sphingomyelinase (ASM) more strongly in cancer pathogenesis, and highlight lysosomal sphingolipid metabolism as a possible weak point for therapeutic targeting in cancer. This review describes the evidence implicating different sphingolipid metabolic enzymes and their products in cancer pathogenesis, and suggests how newer systems-level approaches may improve our overall understanding of how oncogenic transformation reconfigures sphingolipid metabolism.

Metabolomics coupled with multivariate data and pathway analysis on potential biomarkers in gastric ulcer and intervention effects of Corydalis yanhusuo alkaloid

Metabolomics, the systematic analysis of potential metabolites in a biological specimen, has been increasingly applied to discovering biomarkers, identifying perturbed pathways, measuring therapeutic targets, and discovering new drugs. By analyzing and verifying the significant difference in metabolic profiles and changes of metabolite biomarkers, metabolomics enables us to better understand substance metabolic pathways which can clarify the mechanism of Traditional Chinese Medicines (TCM). Corydalis yanhusuo alkaloid (CA) is a major component of Qizhiweitong (QZWT) prescription which has been used for treating gastric ulcer for centuries and its mechanism remains unclear completely. Metabolite profiling was performed by high-performance liquid chromatography combined with time-of-flight mass spectrometry (HPLC/ESI-TOF-MS) and in conjunction with multivariate data analysis and pathway analysis. The statistic software Mass Profiller Prossional (MPP) and statistic method including ANOVA and principal component analysis (PCA) were used for discovering novel potential biomarkers to clarify mechanism of CA in treating acid injected rats with gastric ulcer. The changes in metabolic profiling were restored to their base-line values after CA treatment according to the PCA score plots. Ten different potential biomarkers and seven key metabolic pathways contributing to the treatment of gastric ulcer were discovered and identified. Among the pathways, sphingophospholipid metabolism and fatty acid metabolism related network were acutely perturbed. Quantitative real time polymerase chain reaction (RT-PCR) analysis were performed to evaluate the expression of genes related to the two pathways for verifying the above results. The results show that changed biomarkers and pathways may provide evidence to insight into drug action mechanisms and enable us to increase research productivity toward metabolomics drug discovery.

Galactosylceramide affects tumorigenic and metastatic properties of breast cancer cells as an anti-apoptotic molecule

It was recently proposed that UDP-galactose:ceramide galactosyltransferase (UGT8), enzyme responsible for synthesis of galactosylceramide (GalCer), is a significant index of tumor aggressiveness and a potential marker for the prognostic evaluation of lung metastases in breast cancer. To further reveal the role of UGT8 and GalCer in breast cancer progression, tumorigenicity and metastatic potential of control MDA-MB-231 cells (MDA/LUC) and MDA-MB-231 cells (MDA/LUC-shUGT8) with highly decreased expression of UGT8 and GalCer after stable expression of shRNA directed against UGT8 mRNA was studied in vivo in athymic nu/nu mice. Control MDA/LUC cells formed tumors and metastatic colonies much more efficiently in comparison to MDA/LUC-shUGT8 cells with suppressed synthesis of GalCer after their, respectively, orthotopic and intracardiac transplantation. These findings indicate that UGT8 and GalCer have a profound effect on tumorigenic and metastatic properties of breast cancer cells. In accordance with this finding, immunohistochemical staining of tumor specimens revealed that high expression of UGT8 accompanied by accumulation of GalCer in MDA-MB-231 cells is associated with a much higher proliferative index and a lower number of apoptotic cells in comparison to the MDA/LUC-shUGT8 cells. In addition, it was found that expression of UGT8 in MDA-MB-231 cells increased their resistance to apoptosis induced by doxorubicin in vitro. Therefore, these data suggest that accumulation of GalCer in tumor cells inhibits apoptosis, which would facilitates metastatic cells to survive in the hostile microenvironment of tumor in target organ.

P-glycoprotein mediates drug resistance via a novel mechanism involving lysosomal sequestration

Localization of the drug transporter P-glycoprotein (Pgp) to the plasma membrane is thought to be the only contributor of Pgp-mediated multidrug resistance (MDR). However, very little work has focused on the contribution of Pgp expressed in intracellular organelles to drug resistance. This investigation describes an additional mechanism for understanding how lysosomal Pgp contributes to MDR. These studies were performed using Pgp-expressing MDR cells and their non-resistant counterparts. Using confocal microscopy and lysosomal fractionation, we demonstrated that intracellular Pgp was localized to LAMP2-stained lysosomes. In Pgp-expressing cells, the Pgp substrate doxorubicin (DOX) became sequestered in LAMP2-stained lysosomes, but this was not observed in non-Pgp-expressing cells. Moreover, lysosomal Pgp was demonstrated to be functional because DOX accumulation in this organelle was prevented upon incubation with the established Pgp inhibitors valspodar or elacridar or by silencing Pgp expression with siRNA. Importantly, to elicit drug resistance via lysosomes, the cytotoxic chemotherapeutics (e.g. DOX, daunorubicin, or vinblastine) were required to be Pgp substrates and also ionized at lysosomal pH (pH 5), resulting in them being sequestered and trapped in lysosomes. This property was demonstrated using lysosomotropic weak bases (NH4Cl, chloroquine, or methylamine) that increased lysosomal pH and sensitized only Pgp-expressing cells to such cytotoxic drugs. Consequently, a lysosomal Pgp-mediated mechanism of MDR was not found for non-ionizable Pgp substrates (e.g. colchicine or paclitaxel) or ionizable non-Pgp substrates (e.g. cisplatin or carboplatin). Together, these studies reveal a new mechanism where Pgp-mediated lysosomal sequestration of chemotherapeutics leads to MDR that is amenable to therapeutic exploitation.

Sphingolipid metabolic pathway: an overview of major roles played in human diseases

Sphingolipids, a family of membrane lipids, are bioactive molecules that participate in diverse functions controlling fundamental cellular processes such as cell division, differentiation, and cell death. Given that most of these cellular processes form the basis for several pathologies, it is not surprising that sphingolipids are key players in several pathological processes. This review discusses the role of the sphingolipid metabolic pathway in diabetes, Alzheimer's disease, and hepatocellular carcinoma, with a special emphasis on the changes in gene expression pattern in these disease conditions. For convenience, the sphingolipid metabolic pathway is divided into hypothetical compartments (modules) with each compartment representing a physiological process and changes in gene expression pattern are mapped to each of these modules. It appears that alterations in the gene expression pattern in these disease conditions are biased to manipulate the system in order to result in a particular disease.

Ceramide glycosylation catalyzed by glucosylceramide synthase and cancer drug resistance

Glucosylceramide synthase (GCS), converting ceramide to glucosylceramide, catalyzes the first reaction of ceramide glycosylation in sphingolipid metabolism. This glycosylation by GCS is a critical step regulating the modulation of cellular activities by controlling ceramide and glycosphingolipids (GSLs). An increase of ceramide in response to stresses, such as chemotherapy, drives cells to proliferation arrest and apoptosis or autophagy; however, ceramide glycosylation promptly eliminates ceramide and consequently, these induced processes, thus protecting cancer cells. Further, persistently enhanced ceramide glycosylation can increase GSLs, participating in selecting cancer cells to drug resistance. GCS is overexpressed in diverse drug-resistant cancer cells and in tumors of breast, colon, and leukemia that display poor response to chemotherapy. As ceramide glycosylation by GCS is a rate-limiting step in GSL synthesis, inhibition of GCS sensitizes cancer cells to anticancer drugs and eradicates cancer stem cells. Mechanistic studies indicate that uncoupling ceramide glycosylation can modulate gene expression, decreasing MDR1 through the cSrc/β-catenin pathway and restoring p53 expression via RNA splicing. These studies not only expand our knowledge in understanding how ceramide glycosylation affects cancer cells but also provide novel therapeutic approaches for targeting refractory tumors.

Attenuation of acetic acid-induced gastric ulcer formation in rats by glucosylceramide synthase inhibitors

INTRODUCTION

Ceramide has been suggested to play a role in apoptosis during gastric ulcerogenesis. The present study is designed to investigate whether accumulated ceramide could serve as the effector molecules of ulcer formation in a rat model of acetic acid-induced gastric ulcer.

METHODS

The effect of fumonisin B1, an inhibitor of ceramide synthase, and of d,l,-threo-1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol (PPMP) and N-butyldeoxynojirimycin (NB-DNJ), both inhibitors of glucosylceramide synthase, on the accumulation of ceramide and formation of gastric ulcer were examined in the rat model of acetic acid-induced gastric ulcer.

RESULTS

Fumonisin B1 attenuated acetic acid-induced gastric ulcer formation, associated with a decrease in the number of apoptotic cells. Our results showed that it is neither the C18- nor the C24-ceramide itself, but the respective metabolites that were ulcerogenic, because PPMP and NB-DNJ attenuated gastric mucosal apoptosis and the consequent mucosal damage in spite of their reducing the degradation of ceramide.

CONCLUSION

The ceramide pathway, in particular, the metabolites of ceramide, significantly contributes to acetic acid-induced gastric damage, possibly via enhancing apoptosis. On the other hand, PPMP and NB-DNJ treatment attenuated gastric mucosal apoptosis and ulcer formation despite increasing the ceramide accumulation, suggesting that it was not the ceramides themselves, but their metabolites that contributed to the ulcer formation in the acetic acid-induced gastric ulcer model.

The role of sphingolipids in drug metabolism and transport

INTRODUCTION

Sphingolipids represent a diverse class of lipid molecules. In addition to their function as membrane structural components, they serve as signaling molecules involved in various biological processes such as cell metabolism, growth, differentiation, stress and inflammatory responses and apoptosis. Sphingolipids may modulate the activity and/or expression of cytochrome P450s (CYPs) and transporters, which suggests that they may affect drug metabolism and excretion.

AREAS COVERED

In this review, the authors provide an overview of the properties of sphingolipid structures and metabolism. They also describe the effects of sphingolipids on the activity and expression of CYPs and transporters. In addition, the authors discuss the pathologic conditions where the sphingolipid metabolism is dysregulated particularly in association with inflammation and cancer.

EXPERT OPINION

Sphingolipidomic approaches have become accessible with the aid of advances in analytical technology. Sphingolipid profiles are modified by diseases, genetic disorders or certain drug treatment. The consequent changes in sphingolipid contents may alter the activities of detoxifying enzymes and those associated with cell viability. Since CYPs and transporters play roles in xenobiotics metabolism and excretion, sphingolipidomic information may be of use in understanding drug effect and toxicity.

Sphingolipids and response to chemotherapy

Chemotherapy is frequently used to treat primary or metastatic cancers, but intrinsic or acquired drug resistance limits its efficiency. Sphingolipids are important regulators of various cellular processes including proliferation, apoptosis, differentiation, angiogenesis, stress, and inflammatory responses which are linked to various aspects of cancer, like tumor growth, neoangiogenesis, and response to chemotherapy. Ceramide, the central molecule of sphingolipid metabolism, generally mediates antiproliferative and proapoptotic functions, whereas sphingosine-1-phosphate and other derivatives have opposing effects. Among the variety of enzymes that control ceramide generation, acid or neutral sphingomyelinases and ceramide synthases are important targets to allow killing of cancer cells by chemotherapeutic drugs. On the contrary, glucosylceramide synthase, ceramidase, and sphingosine kinase are other targets driving cancer cell resistance to chemotherapy. This chapter focuses on ceramide-based mechanisms leading to cancer therapy sensitization or resistance which could have some impacts on the development of novel cancer therapeutic strategies.

Sphingolipids: regulators of crosstalk between apoptosis and autophagy

Apoptosis and autophagy are two evolutionarily conserved processes that maintain homeostasis during stress. Although the two pathways utilize fundamentally distinct machinery, apoptosis and autophagy are highly interconnected and share many key regulators. The crosstalk between apoptosis and autophagy is complex, as autophagy can function to promote cell survival or cell death under various cellular conditions. The molecular mechanisms of crosstalk are beginning to be elucidated and have critical implications for the treatment of various diseases, such as cancer. Sphingolipids are a class of bioactive lipids that mediate many key cellular processes, including apoptosis and autophagy. By targeting several of the shared regulators, sphingolipid metabolites differentially regulate the induction of apoptosis and autophagy. Importantly, individual sphingolipid species appear to "switch" autophagy toward cell survival (e.g., sphingosine-1-phosphate) or cell death (e.g., ceramide, gangliosides). This review assesses the current understanding of sphingolipid-induced apoptosis and autophagy to address how sphingolipids mediate the "switch" between the cell survival and cell death. As sphingolipid metabolism is frequently dysregulated in cancer, sphingolipid-modulating agents, or sphingomimetics, have emerged as a novel chemotherapeutic strategy. Ultimately, a greater understanding of sphingolipid-mediated crosstalk between apoptosis and autophagy may be critical for enhancing the chemotherapeutic efficacy of these agents.

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.

Therapeutic potential of targeting ceramide/glucosylceramide pathway in cancer

Sphingolipids including ceramides and its derivatives such as ceramide-1-phosphate, glucosylceramide (GlcCer), and sphingosine-1-phosphate are essential structural components of cell membranes. They now recognized as novel bioeffector molecules which control various aspects of cell growth, proliferation, apoptosis, and drug resistance. Ceramide, the central molecule of sphingolipid metabolism, generally mediates anti-proliferative responses such as inhibition of cell growth, induction of apoptosis, and/or modulation of senescence. There are two major classes of sphingolipids. One of them is glycosphingolipids which are synthesized from the hydrophobic molecule, ceramide. GlcCer, generated by glucosylceramide synthase (GCS) that transfers the glucose from UDP-glucose to ceramide, is an important glycosphingolipid metabolic intermediate. GCS regulates the balance between apoptotic ceramide and antiapoptotic GlcCer. Downregulation or inhibition of GCS results in increased apoptosis and decreased drug resistance. The mechanism underlying the drug resistance which develops with increased glucosylceramide expression is associated with P-glycoprotein. In various types of cancers, overexpression of GCS has been observed which renders GCS a good target for the treatment of cancer. This review summarizes our current knowledge on the structure and functions of glucosylceramide synthase and glucosylceramide and on the roles of glucosylceramide synthase in cancer therapy and drug resistance.

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.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Gene interaction enrichment and network analysis to identify dysregulated pathways and their interactions in complex diseases

BACKGROUND

The molecular behavior of biological systems can be described in terms of three fundamental components: (i) the physical entities, (ii) the interactions among these entities, and (iii) the dynamics of these entities and interactions. The mechanisms that drive complex disease can be productively viewed in the context of the perturbations of these components. One challenge in this regard is to identify the pathways altered in specific diseases. To address this challenge, Gene Set Enrichment Analysis (GSEA) and others have been developed, which focus on alterations of individual properties of the entities (such as gene expression). However, the dynamics of the interactions with respect to disease have been less well studied (i.e., properties of components ii and iii).

RESULTS

Here, we present a novel method called Gene Interaction Enrichment and Network Analysis (GIENA) to identify dysregulated gene interactions, i.e., pairs of genes whose relationships differ between disease and control. Four functions are defined to model the biologically relevant gene interactions of cooperation (sum of mRNA expression), competition (difference between mRNA expression), redundancy (maximum of expression), or dependency (minimum of expression) among the expression levels. The proposed framework identifies dysregulated interactions and pathways enriched in dysregulated interactions; points out interactions that are perturbed across pathways; and moreover, based on the biological annotation of each type of dysregulated interaction gives clues about the regulatory logic governing the systems level perturbation. We demonstrated the potential of GIENA using published datasets related to cancer.

CONCLUSIONS

We showed that GIENA identifies dysregulated pathways that are missed by traditional enrichment methods based on the individual gene properties and that use of traditional methods combined with GIENA provides coverage of the largest number of relevant pathways. In addition, using the interactions detected by GIENA, specific gene networks both within and across pathways associated with the relevant phenotypes are constructed and analyzed.

Reticulon Protein-1C: A New Hope in the Treatment of Different Neuronal Diseases

Reticulons (RTNs) are a group of membrane proteins localized on the ER and known to regulate ER structure and functions. Several studies have suggested that RTNs are involved in different important cellular functions such as changes in calcium homeostasis, ER-stress-mediated cell death, and autophagy. RTNs have been demonstrated to exert a cancer specific proapoptotic function via the interaction or the modulation of specific proteins. Reticulons have also been implicated in different signaling pathways which are at the basis of the pathogenesis of several neurodegenerative diseases. In this paper we discuss the accumulating evidence identifying RTN-1C protein as a promising target in the treatment of different pathologies such as cancer or neurodegenerative disorders.

Sphingolipids in cancer

The bioactive sphingolipids including, ceramide, sphingosine, and sphingosine-1-phosphate (S1P) have important roles in several types of signaling and regulation of many cellular processes including cell proliferation, apoptosis, senescence, angiogenesis, and transformation. Recent accumulating evidence suggests that ceramide- and S1P-mediated pathways have been implicated in cancer development, progression, and chemotherapy. Ceramide mediates numerous cell-stress responses, such as induction of apoptosis and cell senescence, whereas S1P plays pivotal roles in cell survival, migration, and inflammation. These sphingolipids with opposing roles can be interconverted within cells, suggesting that the balance between them is related to cell fate. Importantly, these sphingolipids are metabolically related through actions of enzymes including ceramidases, ceramide synthases, sphingosine kinases, and S1P phosphatases thereby forming a network of metabolically interrelated bioactive lipid mediators whose importance in normal cellular function and diseases is gaining appreciation. In this review, we summarize involvement of sphingolipids and their related enzymes in pathogenesis and therapy of cancer and discuss future directions of sphingolipid field in cancer research.

GCS overexpression is associated with multidrug resistance of human HCT-8 colon cancer cells

Purpose

Multidrug resistance is one of the main impediments to the successful treatment of colon cancer. Glucosylceramide synthase (GCS) which is related to multidrug resistance (MDR) can reduce the level of ceramide and can help cells escape from the ceramide-induced cell apoptosis. However, the underlying mechanism is still unclear.

Methods

The cell proliferation and cell toxicity were measured with Cell Counting Kit-8 (CCK-8). The mRNA levels of GCS and MDR1 were detected by semiquantitative reverse transcription-PCR amplification, the protein levels of GCS, caspase-3 and P-gp proteins were indicated by Western blotting. The apoptosis rates of cells were measured with flow cytometry.

Results

The relative mRNA levels of GCS in HCT-8, HCT-8/VCR, HCT-8/VCR- sh-mock and HCT-8/VCR-sh-GCS were 71.4 ± 1.1%, 95.1 ± 1.2%, 98.2 ± 1.5%, and 66.6 ± 2.1% respectively. The mRNA levels of MDR1 were respectively 61.3 ± 1.1%, 90.5 ± 1.4%, 97.6 ± 2.2% and 56.1 ± 1.2%. The IC50 of Cisplatin complexes were respectively 69.070 ± 0.253 μg/ml, 312.050 ± 1.46 μg/ml, 328.741 ± 5.648 μg/ml, 150.792 ± 0.967 μg/ml in HCT-8, HCT-8/VCR, HCT-8/VCR-sh-mock and HCT-8/VCR-sh-GCS. The protein levels of caspase-3 were 34.2 ± o.6%, 93.0 ± 0.7%, 109.09 ± 0.7%, 42.7 ± 1.3% respectively. The apoptosis rates of cells were 8.77 ± 0.14%, 12.75 ± 0.54%, 15.39 ± 0.41% and 8.49 ± 0.23% respectively.

Conclusion

In conclusion, our research indicated that suppression of GCS restores the sensitivity of multidrug resistance colon cancer cells to drug treatment.

DNA damage induces down-regulation of UDP-glucose ceramide glucosyltransferase, increases ceramide levels and triggers apoptosis in p53-deficient cancer cells

DNA damaging agents typically induce an apoptotic cascade in which p53 plays a central role. However, absence of a p53-mediated response does not necessarily abrogate programmed cell death, due to the existence of p53-independent apoptotic pathways, such as those mediated by the pro-apoptotic molecule ceramide. We compared ceramide levels before and after DNA damage in human osteosarcoma (U2OS) and colon cancer (HCT116) cells that were either expressing or deficient in p53. When treated with mitomycin C, p53-deficient cells, but not p53-expressing cells, showed a marked increase in ceramide levels. Microarray analysis of genes involved in ceramide metabolism identified acid ceramidase (ASAH1, up-regulated), ceramide glucosyltransferase (UGCG, down-regulated), and galactosylceramidase (GALC, up-regulated) as the three genes most affected. Experiments employing pharmacological and siRNA agents revealed that inhibition of UGCG is sufficient to increase ceramide levels and induce cell death. When inhibition of UGCG and treatment with mitomycin C were combined, p53-deficient, but not p53-expressing cells, showed a significant increase in cell death, suggesting that the regulation of sphingolipid metabolism could be used to sensitize cells to chemotherapeutic drugs.

Increase in docetaxel-resistance of ovarian carcinoma-derived RMG-1 cells with enhanced expression of Lewis Y antigen

Epithelial carcinomas of the ovary exhibit the highest mortality rate among gynecologic malignancies. Studies found that the metabolism of glycolipids or carbohydrates is associated with acquirement of anticancer drug-resistance by cancer cells. This study was to characterize possible involvement of Lewis Y (Le(Y)) antigen in the drug-resistance of cancer cells. We transfected the α1,2-fucosyltransferase gene into human ovarian carcinoma-derived RMG-1 cells and established RMG-1-hFUT cells with enhanced expression of Le(Y). We determined the effects of docetaxel on the survival of cells by MTT assaying and observed the apoptosis of cells in the presence of docetaxel by flow cytometric analysis and by transmission electron microscopy. Plasma membranes and intracellular granules in RMG-1-hFUT cells were stained with anti-Le(Y) antibody, the intensity of the staining was higher than that in control cells. The RMG-1-hFUT cells exhibited higher resistance to docetaxel than the control cells with regard to the docetaxel concentration and time course. After treatment with 10 μg/mL docetaxel for 72 h, the control cells, but not RMG-1-hFUT, contained abundant positively stained cell debris due to disintegration of the cytoskeleton. On transmission electron microscopy, although the control cells treated with docetaxel as above showed the following morphology, i.e., absence of villi, cells shrunken in size, pyknosis, agglutinated chromatin and cell buds containing nuclei in the process of apoptosis, the RMG-1-hFUT cells showed only agglutinated chromatin and vacuoles in the cytoplasm. In summary, cells with enhanced expression of Le(Y) were shown to acquire docetaxel-resistance, indicating the possible involvement of glycoconjugates in the drug-resistance.

C6-ceramide and targeted inhibition of acid ceramidase induce synergistic decreases in breast cancer cell growth

The sphingolipid ceramide is known to play a central role in chemo- and radiation-induced cell death. Acid ceramidase (AC) hydrolyzes ceramide, and thus reduces intracellular levels of this proapoptotic lipid. The role of AC as a putative anticancer target is supported by reports of upregulation in prostate cancer and in some breast tumors. In this study, we determined whether the introduction of an AC inhibitor would enhance the apoptosis-inducing effects of C6-ceramide (C6-cer) in breast cancer cells. Cultured breast cancer cells were treated with DM102 [(2R,3Z)-N-(1-hydroxyoctadec-3-en-2-yl)pivalamide, C6-cer, or the combination. Cell viability and cytotoxic synergy were assessed. Activation of apoptotic pathways, generation of reactive oxygen species, and mitochondrial transmembrane potential were determined. DM102 was a more effective AC inhibitor than N-oleoylethanolamine (NOE) and (1R,2R)-2-N-(tetradecanoylamino)-1-(4'-nitrophenyl)-1,3-propandiol (B-13) in MDA-MB-231, MCF-7, and BT-474 cells. As single agents, C6-cer (IC(50) 5-10 μM) and DM102 (IC(50) 20 μM) were only moderately cytotoxic in MDA-MB-231, MCF-7, and SK-BR-3 cells. Co-administration, however, produced synergistic decreases in viability (combination index <0.5) in all cell lines. Apoptosis was confirmed in MDA-MB-231 cells by detection of caspase 3 cleavage and a >3-fold increase in caspase 3/7 activation, PARP cleavage, and a >70% increase in Annexin-V positive cells. C6-cer/DM102 increased ROS levels 4-fold in MDA-MB-231 cells, shifted the ratio of Bax:Bcl-2 to >9-fold that of control cells, and resulted in mitochondrial membrane depolarization. DM102 also increased the synthesis of (3)H-palmitate-labeled long-chain ceramides by 2-fold when C6-cer was present. These data support the effectiveness of targeting AC in combination with exogenous short-chain ceramide as an anticancer strategy, and warrant continued investigation into the utility of the C6-cer/DM102 drug duo in human breast cancer.

Inhibition of acid ceramidase by a 2-substituted aminoethanol amide synergistically sensitizes prostate cancer cells to N-(4-hydroxyphenyl) retinamide

BACKGROUND

The purpose of this study was to determine whether the therapeutic efficacy of fenretinide (4-HPR), a ceramide-generating anticancer agent, could be enhanced in prostate cancer cells by inclusion of a novel synthetic acid ceramidase (AC) inhibitor, DM102, a pivaloylamide of a 2-substituted aminoethanol. In prostate cancer, AC plays a role in progression and resistance to chemotherapy.

METHODS

PC-3 and DU 145 hormone-refractory human prostate cancer cell lines were used. Cells were exposed to 4-HPR, DM102, and combinations; viability, apoptosis, cell migration, ceramide metabolism, and levels of reactive oxygen species (ROS) were assessed.

RESULTS

Single agent 4-HPR and DM102 (2.5-10 µM) were weakly cytotoxic; however, combinations synergistically decreased cell viably to as low as 1.5% of control. N-oleoylethanolamine (NOE), a frequently employed AC inhibitor, was not effective in producing synergy. The 4-HPR/DM102 regimen enhanced caspase activity and increased [(3) H](dihydro)ceramide and ROS levels 6- and 30-fold over control, respectively. The antioxidant vitamin E, but not the de novo ceramide synthesis inhibitor myriocin, partially rescued cells from 4-HPR/DM102 cytotoxicity. The 4-HPR/DM102 combination also elicited synergistic cytotoxicity in DU 145 cells, another human hormone-refractory prostate cancer cell line.

CONCLUSION

This study shows that 4-HPR cytotoxicity is enhanced in a synergistic fashion by inclusion of the AC inhibitor DM102, by a mechanism that enlists generation of ROS, and thus provides a system to raise 4-HPR therapeutic potential. The role of ceramide however in the cytotoxic response is not clear, as blocking ceramide generation failed to rescue PC-3 cells from 4-HPR/DM102 cytotoxicity.

Glucosylceramide synthase, a factor in modulating drug resistance, is overexpressed in metastatic breast carcinoma

Drug resistance causes treatment failure in approximately 50% of breast cancer patients with chemotherapy. Overexpression of glucosylceramide synthase (GCS) confers drug resistance in cancer cells, and suppression of GCS sensitizes cancers to chemotherapy in preclinical studies. Thus, GCS becomes a potential target to reverse drug resistance; however, little is known about GCS expression levels in normal tissues and whether GCS overexpression is associated with metastatic cancers. Herewith, we report our studies in GCS expression levels and breast cancer from patients. GCS levels were analyzed using cancer profiling arrays, breast cancer histo-arrays and quantitative RT-PCR in tumor tissues. We found that breast (18 exp. index) and other hormone-dependent organs (testis, cervix, ovary, prostate) displayed the lowest levels of GCS mRNA, whereas liver (52 exp. index) and other organs (kidney, bladder, stomach) displayed the highest levels of GCS. GCS mRNA levels were significantly elevated in tumors of breast, cervix, rectum and small intestine, as compared to each paired normal tissue. In mammary tissue, GCS overexpression was detected in breast cancers with metastasis, but not in benign fibroadenoma or primary tumors. GCS overexpression was coincident with HER2 expression (γ2=0.84) in ER-negative breast adenocarcinoma. In tumor specimens, GCS mRNA was elevated by 4-fold and significantly associated with stage III (5/7), lymph node-positive (7/8) and estrogen receptor-positive breast cancers (7/9). GCS expression was significantly and selectively elevated in breast cancer, in particular in metastatic disease. GCS overexpression was highly associated with ER-positive and HER2-positive breast cancer with metastasis. Although a small study, these data suggest that GCS may be a prognostic indicator and potential target for the treatment of chemotherapy-refractory breast cancer.

P-glycoprotein antagonists confer synergistic sensitivity to short-chain ceramide in human multidrug-resistant cancer cells

P-glycoprotein (P-gp) antagonists inhibit ceramide metabolism at the juncture of glycosylation. The purpose of this study was to test whether targeting P-gp would be a viable alternative to targeting glucosylceramide synthase (GCS) for enhancing ceramide cytotoxicity. A2780 wild-type, and multidrug-resistant 2780AD and NCI/ADR-RES human ovarian cancer cell lines and the cell-permeable ceramide analog, C6-ceramide (C6-cer), were employed. Compared to P-gp-poor A2780 cells, P-gp-rich 2780AD cells converted 3.7-fold more C6-cer to nontoxic C6-glucosylceramide (C6-GC), whereas cell-free GCS activities were equal. 2780AD cells displayed resistance to C6-cer (10 μM) that was reversed by inclusion of the P-gp antagonist tamoxifen (5 μM) but not by inclusion of a GCS inhibitor. Co-administration of C6-cer and P-gp antagonists was also effective in NCI/ADR-RES cells. For example, C6-cer, VX-710 (Biricodar), and cyclosporin A (cyc A) exposure resulted in viabilities of ~90% of control; however, C6-cer/VX-710 and C6-cer/cyc A additions were synergistic and resulted in viabilities of 22% and 17%, respectively. Further, whereas C6-ceramide and cyc A imparted 1.5- and 0-fold increases in caspase 3/7 activity, the combination produced a 3.5-fold increase. Although the upstream elements of cell death have not been elucidated, the novel C6-ceramide/P-gp antagonist combination merits further study and assessment of clinical translational potential.

Ceramide glucosyltransferase of the nematode Caenorhabditis elegans is involved in oocyte formation and in early embryonic cell division

Ceramide glucosyltransferase (Ugcg) [uridine diphosphate (UDP)-glucose:N-acylsphingosine D-glucosyltransferase or UDP-glucose ceramide glucosyltransferase (GlcT): EC 2.4.1.80] catalyzes formation of glucosylceramide (GlcCer) from ceramide and UDP-glucose. There is only one Ugcg gene in the mouse genome, which is essential in embryogenesis and brain development. The nematode Caenorhabditis elegans has three Ugcg genes (cgt-1, cgt-2 and cgt-3), and double RNAi of the cgt-1 and cgt-3 genes results in lethality at the L1 larval stage. In this study, we isolated knockout worms for the three genes and characterized the gene functions. Each gene product showed active enzymatic activity when expressed in GM95 cells deficient in glycosphingolipids (GSLs). When each gene function was disrupted, the brood size of the animal markedly decreased, and abnormal oocytes and multinucleated embryos were formed. The CGT-3 protein had the highest Ugcg activity, and knockout of its gene resulted in the severest phenotype. When cgt-3 RNAi was performed on rrf-1 worms lacking somatic RNAi machinery but with intact germline RNAi machinery, a number of abnormal oocytes and multinucleated eggs were observed, although the somatic phenotype, i.e., L1 lethal effects of cgt-1/cgt-3 RNAi, was completely suppressed. Cell surface expression of GSLs and sphingomyelin, which are important components of membrane domains, was affected in the RNAi-treated embryos. In the embryos, an abnormality in cytokinesis was also observed. From these results, we concluded that the Ugcg gene is indispensable in the germline and that an ample supply of GlcCer is needed for oocytes and fertilized eggs to maintain normal membranes and to proceed through the normal cell cycle.

Apoptotic sphingolipid ceramide in cancer therapy

Apoptosis, also called programmed cell death, is physiologically and pathologically involved in cellular homeostasis. Escape of apoptotic signaling is a critical strategy commonly used for cancer tumorigenesis. Ceramide, a derivative of sphingolipid breakdown products, acts as second messenger for multiple extracellular stimuli including growth factors, chemical agents, and environmental stresses, such as hypoxia, and heat stress as well as irradiation. Also, ceramide acts as tumor-suppressor lipid because a variety of stress stimuli cause apoptosis by increasing intracellular ceramide to initiate apoptotic signaling. Defects on ceramide generation and sphingolipid metabolism are developed for cancer cell survival and cancer therapy resistance. Alternatively, targeting ceramide metabolism to correct these defects might provide opportunities to overcome cancer therapy resistance.

Ceramide galactosyltransferase (UGT8) is a molecular marker of breast cancer malignancy and lung metastases

BACKGROUND

It was shown recently on the level of gene expression that UGT8, coding UDP-galactose:ceramide galactosyltransferase, is one of six genes whose elevated expression correlated with a significantly increased the risk of lung metastases in breast cancer patients. In this study primary tumours and their lung metastases as well as breast cancer cell lines were analysed for UGT8 expression at the protein level.

METHODS

Expression of UGT8 in breast cancer tissue specimens and breast cancer cell lines was analysed using IHC, real-time PCR and Western blotting.

RESULTS

Comparison of the average values of the reaction intensities (IRS scale) showed a significant difference in UGT8 expression between (1) primary and metastatic tumours (Mann-Whitney U, P<0.05), (2) tumours of malignancy grades G3 and G2 (Mann-Whitney U, P<0.01) as well as G3 and G1 (Mann-Whitney U, P<0.001) and (3) node-positive and node-negative tumours (Mann-Whitney U, P<0.001). The predictive ability of increased expression of UGT8 was validated at the mRNA level in three independent cohorts of breast cancer patients (721). Similarly, breast cancer cell lines with the 'luminal epithelial-like' phenotype did not express or weakly expressed UGT8, in contrast to malignant, 'mesenchymal-like,' cells forming metastases in nude mice.

CONCLUSION

Our data suggest that UGT8 is a significant index of tumour aggressiveness and a potential marker for the prognostic evaluation of lung metastases in breast cancer.

Expanding spectrum of the association between Type 1 Gaucher disease and cancers: a series of patients with up to 3 sequential cancers of multiple types--correlation with genotype and phenotype

In Gaucher disease (GD), inherited deficiency of lysosomal glucocerebrosidase due to mutations in GBA1 gene results in accumulation of glucosylceramide in tissue macrophages, systemic macrophage activation, and a complex multisystemic phenotype. We and others have reported an increased risk of multiple myeloma and other malignancies in non-neuronopathic Type 1 GD (GD1). Here, we describe a subset of GD1 patients with multiple malignancies. In our cohort of 403 patients with GD1, nine patients (2.2%) developed two or three different types of cancers either consecutively or simultaneously. Patients were characterized by age at diagnosis of GD1, GBA1 genotype, disease severity, age at cancer diagnosis, enzyme replacement therapy (ERT) status, and splenectomy status. Of the nine patients, six developed two types of malignancies and three had three cancers each. Overall, the hematologic malignancies comprised lymphoma/leukemia (4) and multiple myeloma (4). Nonhematologic malignancies included colon (2), lung (2), thyroid (2), and prostate cancer (1). Of the seven patients who received ERT, the first cancer was diagnosed before initiation of ERT in all but one. Asplenic patients were more likely to have single or multiple cancers compared with patients with intact spleens (P < 0.0072 and P < 0.0203, respectively). Our data strengthen the association of GD1 and cancer and suggest that patients may be at risk of developing multiple malignancies. We found an association between splenectomy and multiple cancers in GD1. It will be of interest to determine whether timely ERT and declining rates of splenectomy will translate into declining rates of multiple and single cancers.

Metabolism of short-chain ceramide by human cancer cells--implications for therapeutic approaches

Due to recent use of short-chain ceramides in preclinical studies, we characterized C6-ceramide metabolism in cancer cell lines and assessed metabolic junctures for enhancing efficacy. MDA-MB-231 breast cancer cells decreased the amount of C6-ceramide metabolized to C6-sphingomyelin (C6-SM) and increased the amount metabolized to C6-glucosylceramide (C6-GC) in response to increasing concentrations. A similar trend was seen in DU-145 (prostate cancer), PANC-1 (pancreatic cancer), and LoVo (colorectal cancer) cells. KG-1 leukemia cells favored C6-SM synthesis at low (0.6muM) and high-dose (12muM) C6-ceramide. Partnering C6-ceramide with tamoxifen, a P-glycoprotein antagonist that impedes ceramide glycosylation, was an effective regimen for enhancing cytotoxicity in cells. Experiments to assess the mechanism of cell death using KG-1 cells showed that tamoxifen inhibited synthesis of C6-GC and C6-SM from C6-ceramide by 80% and 50%, respectively, which was accompanied by enhanced apoptosis. Radiolabeling of KG-1 cells with [(3)H]palmitic acid produced a 2-fold increase in (3)H-long-chain ceramides when unlabeled C6-ceramide was added and a 9-fold increase when C6-ceramide and tamoxifen were added. The increase in (3)H-palmitate radiolabeling of long-chain ceramides was blocked by inclusion of a ceramide synthase inhibitor; however, inhibiting synthesis of long-chain ceramide did not rescue cells. These studies show that tamoxifen enhances the apoptotic effects of C6-ceramide. The proposed mechanism involves blocking short-chain ceramide anabolism to favor hydrolysis and generation of sphingosine. We propose that use of tamoxifen and other P-glycoprotein antagonists can be an effective means for enhancing cytotoxic potential of short-chain ceramides in the treatment of cancer.

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.

Augmentation of therapeutic efficacy in drug-resistant tumor models using ceramide coadministration in temporal-controlled polymer-blend nanoparticle delivery systems

The development of multidrug resistance (MDR) is a major hindrance to cancer eradication as it renders tumors unresponsive to most chemotherapeutic treatments and is associated with cancer resurgence. This study describes a novel mechanism to overcome MDR through a polymer-blend nanoparticle platform that delivers a combination therapy of C6-ceramide (CER), a synthetic analog of an endogenously occurring apoptotic modulator, together with the chemotherapeutic drug paclitaxel (PTX), in a single formulation. The PTX/CER combination therapy circumvents another cellular mechanism whereby MDR develops, by lowering the threshold for apoptotic signaling. In vivo studies in a resistant subcutaneous SKOV3 human ovarian and in an orthotopic MCF7 human breast adenocarcinoma xenograft showed that the PTX and CER nanoparticle combination therapy reduced the final tumor volume at least twofold over treatment with the standard PTX therapy alone. The study also revealed that the cotherapy accomplished this enhanced efficacy by generating an enhancement in apoptotic signaling in both tumor types. Additionally, acute evaluation of safety with the combination therapy did not show significant changes in body weight, white blood cell counts, or liver enzyme levels. The temporal-controlled nanoparticle delivery system presented in this study allows for a simultaneous delivery of PTX + CER in breast and ovarian tumor model drug, leading to a modulation of the apoptotic threshold. This strategy has tremendous potential for effective treatment of refractory disease in cancer patients.

Immunosuppressors as multidrug resistance reversal agents

Multidrug-resistance (MDR) is the major reason for failure of cancer therapy. ATP-binding cassette (ABC) transporters contribute to drug resistance via ATP-dependent drug efflux. P-glycoprotein (Pgp), which is encoded by MDR1 gene, confers resistance to certain anticancer agents. The development of agents able to modulate MDR mediated by Pgp and other ABC transporters remained a major goal for the past 20 years. The calcium blocker verapamil was the first drug shown to be a modulator of Pgp, and since many different chemical compounds have been shown to exert the same effect in vitro by blocking Pgp activity. These included particularly immunosuppressors. Cyclosporin A (CSA) was the first immunosuppressor that have been shown to modulate Pgp activity in laboratory models and entered very early into clinical trials for reversal of MDR. The proof of reversing activity of CSA was found in phase II studies with myeloma and acute leukemia. In phase III studies, the results were less convincing regarding the response rate, progression-free survival, and overall survival, which were detected in advanced refractory myeloma. The non-immunosuppressive derivative PSC833 (valspodar) was subsequently developed. This compound showed tenfold higher potency in reversal of MDR mediated by Pgp. However, pharmacokinetic interactions required reductions in the dose of the concurrently administered anticancer agents. The pharmacokinetic interactions were likely because of decreased clearance of the anticancer agents, possibly as a result of Pgp inhibition in organs such as the gastrointestinal tract and kidney, as well as inhibition of cytochrome P450. Finally, CSA and PSC833 have been shown also to modulate the ceramide metabolism which stands as second messenger of anticancer agent-induced apoptosis. In fact, CSA and PSC833 are also able to respectively inhibit ceramide glycosylation and stimulate de novo ceramide synthesis. This could enhance the cellular level of ceramide and potentiate apoptosis induced by some anticancer agents.

Glucosylceramide in humans

Glucosylceramide has a unique and often ambiguous role in mammalian cells. Activation of glucosylceramide synthase, the enzyme that places a glucosyl moiety onto ceramide, is the first pathway-committed step to the production of more complex glycosphingolipids such as lactosylceramide and gangliosides. Alterations in the level of glucosylceramide are noted in cells and tissues in response to cardiovascular disease, diabetes, skin disorders and cancer. Overall, upregulation of glucosylceramide offers cellular protection and primes certain cells for proliferation. However, prolonged overabundance of glucosylceramide is detrimental, as seen in Gaucher disease in humans.

GCS induces multidrug resistance by regulating apoptosis-related genes in K562/AO2 cell line

We have previously shown that the expression of glucosylceramide synthase (GCS) gene in drug-resistant K562/AO2 human leukemia cell was higher than that in drug-sensitive K562 cell, and the sensitivity to adriamycin of K562/AO2 cell was enhanced by inhibiting GCS. It is concluded that the overexpression of GCS gene is one of the reasons which lead to multidrug resistance (MDR) of leukemia cell. Meanwhile, we also found that higher expression of Bcl-2 gene and protein were exhibited in K562/AO2 cell compared with K562 cell. Basing on this, we hypothesized that the high expression of GCS gene which results in MDR of leukemia cell is correlated with Bcl-2 signal transduction. In order to validate the hypothesis, the inhibition of GCS gene in K562/AO2 cell was observed by using chemical suppressor PPMP and siRNA targeted at GCS, and applying RT-PCR and flow cytometry, the expression levels of apoptosis-related gene Bcl-2 and Bax were analyzed before and after inhibiting GCS gene in K562/AO2 cell. The results demonstrated that the gene and protein of Bcl-2 in K562/AO2 cell were both down-regulated significantly after GCS gene being inhibited; however, the Bax mRNA expression had no apparent change in different groups. This suggested that GCS gene may contributed to MDR of human leukemia cell K562/AO2 by Bcl-2 signal transduction.

A new mixed-backbone oligonucleotide against glucosylceramide synthase sensitizes multidrug-resistant tumors to apoptosis

Enhanced ceramide glycosylation catalyzed by glucosylceramide synthase (GCS) limits therapeutic efficiencies of antineoplastic agents including doxorubicin in drug-resistant cancer cells. Aimed to determine the role of GCS in tumor response to chemotherapy, a new mixed-backbone oligonucleotide (MBO-asGCS) with higher stability and efficiency has been generated to silence human GCS gene. MBO-asGCS was taken up efficiently in both drug-sensitive and drug-resistant cells, but it selectively suppressed GCS overexpression, and sensitized drug-resistant cells. MBO-asGCS increased doxorubicin sensitivity by 83-fold in human NCI/ADR-RES, and 43-fold in murine EMT6/AR1 breast cancer cells, respectively. In tumor-bearing mice, MBO-asGCS treatment dramatically inhibited the growth of multidrug-resistant NCI/ADR-RE tumors, decreasing tumor volume to 37%, as compared with scrambled control. Furthermore, MBO-asGCS sensitized multidrug-resistant tumors to chemotherapy, increasing doxorubicin efficiency greater than 2-fold. The sensitization effects of MBO-asGCS relied on the decreases of gene expression and enzyme activity of GCS, and on the increases of C(18)-ceramide and of caspase-executed apoptosis. MBO-asGCS was accumulation in tumor xenografts was greater in other tissues, excepting liver and kidneys; but MBO-asGCS did not exert significant toxic effects on liver and kidneys. This study, for the first time in vivo, has demonstrated that GCS is a promising therapeutic target for cancer drug resistance, and MBO-asGCS has the potential to be developed as an antineoplastic agent.

Targeting glucosylceramide synthase downregulates expression of the multidrug resistance gene MDR1 and sensitizes breast carcinoma cells to anticancer drugs

Drug resistance in breast cancer remains a major cause for the failure of chemotherapy. Glucosylceramide synthase (GCS) plays an important role in multidrug resistance (MDR) in breast cancer. P-glycoprotein (P-gp) also confers a cross-resistance of many unrelated drugs. In this study, we studied the MDR effect and potential mechanisms of breast cancer after constructing permanent breast cancer cell lines with GCS knockout by using recombinant vectors targeting GCS (pSUPER-GCSshRNAs). The GCSshRNA stably transfected cells were successfully established and significant lower levels of GCS mRNA and protein expression were confirmed. In in vitro experiments, the GCSshRNA stably transfected cells showed a significantly reduced level of MDR1 and P-gp expression and decreased drug efflux ability. Reduced level of GCS expression conveyed a significant reversal of drug resistance by MTT assay and increased caspase-3 activity. In in vivo experiments by using nude mice with xenograft tumors, a significant inhibition of tumor growth was observed after comparing with the control group. Furthermore, enhanced response of chemotherapy was acquired by reduced expression of GCS as well as MDR1 in vivo. In conclusion, GCSshRNA could efficiently suppress GCS and MDR1 expression in vitro and in vivo and these findings may be used as one of the methods to reverse MDR in breast cancer.

Cholesterol-rich fluid membranes solubilize ceramide domains: implications for the structure and dynamics of mammalian intracellular and plasma membranes

A uniquely sensitive method for ceramide domain detection allowed us to study in detail cholesterol-ceramide interactions in lipid bilayers with low (physiological) ceramide concentrations, ranging from low or no cholesterol (a situation similar to intracellular membranes, such as endoplasmic reticulum) to high cholesterol (similar to mammalian plasma membrane). Diverse fluorescence spectroscopy and microscopy experiments were conducted showing that for low cholesterol amounts ceramide segregates into gel domains that disappear upon increasing cholesterol levels. This was observed in different raft (sphingomyelin/cholesterol-containing) and non-raft (sphingomyelin-absent) membranes, i.e. mimicking different types of cell membranes. Cholesterol-ceramide interactions have been described mainly as raft sphingomyelin-dependent. Here sphingomyelin independence is demonstrated. In addition, ceramide-rich domains re-appear when either cholesterol is converted by cholesterol oxidase to cholestenone or the temperature is decreased. Ceramide is more soluble in cholesterol-rich fluid membranes than in cholesterol-poor ones, thereby increasing the chemical potential of cholesterol. Ceramide solubility depends on the average gel-fluid transition temperature of the remaining membrane lipids. The inability of cholestenone-rich membranes to dissolve ceramide gel domains shows that the cholesterol ordering and packing properties are fundamental to the mixing process. We also show that the solubility of cholesterol in ceramide domains is low. The results are rationalized by a ternary phospholipid/ceramide/cholesterol phase diagram, providing the framework for the better understanding of biochemical phenomena modulated by cholesterol-ceramide interactions such as cholesterol oxidase activity, lipoprotein metabolism, and lipid targeting in cancer therapy. It also suggests that the lipid compositions of different organelles are such that ceramide gel domains are not formed unless a stress or pathological situation occurs.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

Glucosylceramide synthase inhibitors sensitise CLL cells to cytotoxic agents without reversing P-gp functional activity

Malignant B-cells from most chronic lymphocytic leukaemia (CLL) patients over-express MDR1 encoded P-glycoprotein (P-gp) multidrug efflux pump. Inhibition of glucosylceramide (GC) synthesis has been shown in cell lines to correlate with the expression and function of P-gp and sensitise cancer cells to cytotoxic agents. We investigated the hypothesis that reducing intracellular GC levels will reduce P-gp expression in malignant cells from CLL patients. We studied the ability of glucosylceramide synthase (GCS) inhibitors N-butyl-deoxygalactonojirimycin (OGB-1) and N-nonyl-deoxygalactonojirimycin (OGB-2) to sensitise CLL cells to conventional cytotoxic drug 2-chlorodeoxyadenosine (CdA) and the cytostatic drugs chlorambucil and fludarabine. The effect on P-gp activity was analysed using the calcein-AM accumulation assay where a multidrug activity factor (MAF) of >10 in the presence of a P-gp inhibitor denotes P-gp functional activity. The P-gp over-expressing cell line CEM-VLB showed a MAF value of 96.4 with the P-gp inhibitor Z.3HCL, which fell to 15.7 after co-incubation with OGB-1 and 45.9 with OGB-2. The IC(50) for vincristine fell from >10 microg/ml to 55.5 ng/ml in the presence of OGB-2. In P-gp(+ve) peripheral blood mononuclear cells from three normal volunteers, the mean MAF values for Z.3HCL, OGB-1 and OGB-2 were 23.86, 1.83 and 16.2 respectively. In 9/13 CLL samples the mean P-gp functional activity was 22.15 and P-gp was over-expressed in 12/13 samples. However, the MAF value with OGB-1 and OGB-2 was <10. Nevertheless, sensitisation in CLL cells was observed by a reduction in the IC(50) in the presence of OGB-1 and OGB-2 with the conventional drugs. We conclude that although GCS inhibitors sensitize CLL cells to cytotoxic and cytostatic drugs, they do not appear to have any effect on P-gp functional activity.

De novo ceramide synthesis is responsible for the anti-tumor properties of camptothecin and doxorubicin in follicular thyroid carcinoma

Doxorubicin and camptothecin are two cytotoxic chemotherapeutic agents triggering apoptosis in various cancer cells, including thyroid carcinoma cells. Recent studies revealed a critical role of ceramide in chemotherapy and suggested that anti-cancer drugs may kill tumor cells through sphingomyelinase activation. However, in comparison to sphingomyelin hydrolysis, the relative involvement of de novo ceramide synthesis remained poorly explored and highly controversial. Here, we evidenced that both doxorubicin and camptothecin triggered ceramide accumulation in thyroid carcinoma cells. We demonstrated that ceramide increase occurred via the de novo pathway without neither acidic nor neutral sphingomyelinase contribution. Interestingly, de novo ceramide generation was responsible for the drug-induced malignant cell apoptosis through a caspase-3-dependent pathway and a decrease of thrombospondin amount. Furthermore, blocking ceramide metabolism by inhibiting glucosylceramide synthase strengthened the camptothecin and doxorubicin-dependent effects. Altogether, we evidenced that de novo ceramide synthesis mediates the anti-tumor properties of doxorubicin and camptothecin in thyroid carcinoma and suggested that glucosylation of ceramide may contribute to the drug-resistance phenotype in thyroid malignancies.

N-(4-Hydroxyphenyl)retinamide increases dihydroceramide and synergizes with dimethylsphingosine to enhance cancer cell killing

Fenretinide [N-(4-hydroxyphenyl)retinamide (4-HPR)] is cytotoxic in many cancer cell types. Studies have shown that elevation of ceramide species plays a role in 4-HPR cytotoxicity. To determine 4-HPR activity in a multidrug-resistant cancer cell line as well as to study ceramide metabolism, MCF-7/AdrR cells (redesignated NCI/ADR-RES) were treated with 4-HPR and sphingolipids were analyzed. TLC analysis of cells radiolabeled with [3H]palmitic acid showed that 4-HPR elicited a dose-responsive increase in radioactivity migrating in the ceramide region of the chromatogram and a decrease in cell viability. Results from liquid chromatography/electrospray tandem mass spectrometry revealed large elevations in dihydroceramides (N-acylsphinganines), but not desaturated ceramides, and large increases in complex dihydrosphingolipids (dihydrosphingomyelins, monohexosyldihydroceramides), sphinganine, and sphinganine 1-phosphate. To test the hypothesis that elevation of sphinganine participates in the cytotoxicity of 4-HPR, cells were treated with the sphingosine kinase inhibitor d-erythro-N,N-dimethylsphingosine (DMS), with and without 4-HPR. After 24 h, the 4-HPR/DMS combination caused a 9-fold increase in sphinganine that was sustained through +48 hours, decreased sphinganine 1-phosphate, and increased cytotoxicity. Increased dihydrosphingolipids and sphinganine were also found in HL-60 leukemia cells and HT-29 colon cancer cells treated with 4-HPR. The 4-HPR/DMS combination elicited increased apoptosis in all three cell lines. We propose that a mechanism of 4-HPR-induced cytotoxicity involves increases in dihydrosphingolipids, and that the synergy between 4-HPR and DMS is associated with large increases in cellular sphinganine. These studies suggest that enhanced clinical efficacy of 4-HPR may be realized through regimens containing agents that modulate sphingoid base metabolism.