Formation of ceramide-enriched domains in lipid particles enhances the binding of apolipoprotein E

We investigated the interaction between apolipoprotein E (apoE) and ceramide (CER)-enriched domains on the particles, by using lipid emulsions containing sphingomyelin (SM) or CER as model particles of lipoproteins. The sphingomyelinase (SMase)-induced aggregation of emulsion particles was prevented by apoE. CER increased the amount of apoE bound to emulsion particles. The confocal images of CER-containing large emulsions with two fluorescent probes showed three-dimensional microdomains enriched in CER. SMase also induced the formation of CER-enriched domains. We propose apoE prefers to bind on CER-enriched domains exposed on particle surface, and thus inhibits the aggregation or fusion of the particles.

Positively charged ceramide is a potent inducer of mitochondrial permeabilization

Ceramide-induced cell death is thought to be mediated by change in mitochondrial function, although the precise mechanism is unclear. Proposed models suggest that ceramide induces cell death through interaction with latent binding sites on the outer or inner mitochondrial membranes, followed by an increase in membrane permeability, as an intermediate step in ceramide signal propagation. To investigate these models, we developed a new generation of positively charged ceramides that readily accumulate in isolated and in situ mitochondria. Accumulated, positively charged ceramides increased inner membrane permeability and triggered release of mitochondrial cytochrome c. Furthermore, the positively charged ceramide-induced permeability increase was suppressed by cyclosporin A (60%) and 1,3-dicyclohexylcarbodiimide (90%). These observations suggest that the inner membrane permeability increase is due to activation of specific ion transporters, not the generalized loss of lipid bilayer barrier functions. The difference in sensitivity of ceramide-induced ion fluxes to inhibitors of mitochondrial transporters suggests activation of at least two transport systems: the permeability transition pore and the electrogenic H(+) channel. Our results indicate the presence of specific ceramide targets in the mitochondrial matrix, the occupation of which triggers permeability alterations of the inner and outer mitochondrial membranes. These findings also suggest a novel therapeutic role for positively charged ceramides.

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

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

Endothelial apoptosis induced by inhibition of integrins alphavbeta3 and alphavbeta5 involves ceramide metabolic pathways

Matrix ligation of integrins alphavbeta3/alphavbeta5 is critical for endothelial survival and angiogenesis. We have previously shown that ceramide, a proapoptotic lipid second messenger, increases during endothelial anoikis (detachment-induced apoptosis). We now show that RGDfV, an integrin alphavbeta3/alphavbeta5 cyclic function-blocking peptide, increased ceramide and decreased sphingomyelin in human brain microvascular endothelial cells (HBMECs) plated on vitronectin, suggesting that sphingomyelin hydrolysis contributes to RGDfV-induced ceramide increase. Desipramine and imipramine, inhibitors of acid sphingomyelinase (ASMase), suppressed RGDfV-induced ceramide increase. Importantly, desipramine, imipramine, and a third ASMase inhibitor, SR33557, but not inhibitors of neutral sphingomyelinase, suppressed RGDfV-induced apoptosis, suggesting that ASMase was required for integrin-mediated apoptosis. Myriocin, an inhibitor of de novo ceramide synthesis, had no effect on RGDfV-induced HBMEC apoptosis. Interestingly, ASMase inhibitors also suppressed the RGDfV-induced loss of spreading on vitronectin. RGDfV induced a similar increase in ceramide and apoptosis in HBMECs on poly-l-lysine or vitronectin, although cells detached only from vitronectin, indicating that cell detachment was not required for RGDfV-induced apoptosis. Our results suggest involvement of ASMase and ceramide in endothelial apoptosis induced by inhibition of integrins alphavbeta3/alphavbeta5, and propose a novel molecular mechanism for the antiangiogenic effect of RGDfV.

Cross-talk between phosphatidic acid and ceramide during ethanol-induced apoptosis in astrocytes

Background

Ethanol inhibits proliferation in astrocytes, an effect that was recently linked to the suppression of phosphatidic acid (PA) formation by phospholipase D (PLD). The present study investigates ethanol's effect on the induction of apoptosis in astrocytes and the formation of ceramide, an apoptotic signal. Evidence is presented that the formation of PA and ceramide may be reciprocally linked during ethanol exposure.

Results

In cultured rat cortical astrocytes, ethanol (0.3–1 %, v/v) induced nuclear fragmentation and DNA laddering indicative of apoptosis. Concomitantly, in cells prelabeled with [³H]-serine, ethanol caused a dose-dependent, biphasic increase of the [³H]-ceramide/ [³H]-sphingomyelin ratio after 1 and 18 hours of incubation. As primary alcohols such as ethanol and 1-butanol were shown to inhibit the phospholipase D (PLD)-mediated formation of PA, a mitogenic lipid messenger, we tested their effects on ceramide formation. In astrocytes prelabeled with [³H]-serine, ethanol and 1-butanol, in contrast to t-butanol, significantly increased the formation of [³H]-ceramide. Moreover, exogenous PA, added to transiently permeabilized astrocytes, suppressed ethanol-induced [³H]-ceramide formation. Vice versa, addition of C₂-ceramide to astrocytes inhibited PLD activity induced by serum or phorbol ester.

Conclusion

We propose that the formation of ceramide in ethanol-exposed astrocytes is secondary to the disruption of phospholipase D signaling. Ethanol reduces the PA:ceramide ratio in fetal astrocytes, a mechanism which likely participates in ethanol-induced glial apoptosis during brain development.

Aromatase inhibition by 15-deoxy-prostaglandin J(2) (15-dPGJ(2)) and N-(4-hydroxyphenyl)-retinamide (4HPR) is associated with enhanced ceramide production

Inhibition of aromatase activity is an established endocrine therapy in the treatment of hormone-dependent breast cancer. Recent studies on aromatase inhibition by the synthetic retinoid 4HPR, also known as fenretinide, and the PPARgamma agonist 15-dPGJ(2) have implicated a direct receptor-independent, redox-sensitive mechanism of action. The signalling molecule ceramide has also been previously implicated as a negative regulator of aromatase activity. In the present study, we have investigated a potential mediatory role for this sphingolipid during aromatase inhibition by fenretinide and 15-dPGJ(2) in the breast cancer cell line MDA MB 231 and JEG-3 choriocarcinoma cells. 4HPR and 15-dPGJ(2) caused a dose-dependent inhibition of aromatase activity associated with an increase in ceramide production. Both these actions were redox-sensitive as demonstrated by their abrogation in the presence of the anti-oxidant N-acetylcysteine. Exogenous ceramide analogue mimicked these inhibitory actions on aromatase, but in a redox-independent manner. Blockade of the de novo ceramide production pathway by fumonisin B(1) or myriocin inhibited the ceramide responses, but did not prevent aromatase inhibition by 15-dPGJ(2) or 4HPR. This study highlights a potential role for aromatase inhibition and the stress-response signal ceramide during the therapeutic actions of 15-dPGJ(2) and 4HPR in breast cancer treatment. However, these data do not support a mediatory role for this sphingolipid during aromatase inhibition by these agents.

Regulation of alternative splicing of caspase-2 through an intracellular signaling pathway in response to pro-apoptotic stimuli

Alternative splicing is an important mechanism in the generation of functionally distinct products from the same gene. Some apoptosis-regulating genes also undergo alternative splicing, generating splice variants that antagonzie normal transcripts on apoptosis. For example, caspase-2 is alternatively spliced, leading to exon 9-lacking caspase-2L (proapoptotic) and exon 9-containing caspase-2S (antiapoptotic) transcripts. Serine-arginine splicing factor proteins (SR proteins) are highly conserved and required for constitutive and alternative messenger RNA (mRNA) splicing. Their activity is regulated by reversible phosphorylation on serine residue. During apoptosis, many functional molecules undergo posttranslational modification, including phosphorylation, dephosphorylation, and caspase cleavage. In this study, we investigated the effect of proapoptotic stimuli on alternative splicing of caspase-2 mRNA in U937 cells. U937 cells were simulated with etoposide, staurosporine, pacritaxel, or cyclohexamide. We analzyed the alternative splicing of caspase-2 mRNA using reverse transcription-polymerase chain reaction. Etoposide, staurosporine, pacritaxel, and cyclohexamide treatment promoted exon-9 inclusion, increasing the ratio of caspase-2S to caspase-2L in a time-dependent manner. Pretreatment with calyculin A, an inhibitor of protein phosphatase-1, blocked etoposide-induced alternative splicing of caspase-2 mRNA. Furthermore, pretreatment of U937 cells with fumonisin B1, an inhibitor of ceramide synthase, also blocked alternative splicing of caspase-2 mRNA. These data demonstrate that endogenous ceramide generation and subsequent phosphatase activation during apoptosis are key steps in the alternative splicing of caspase-2 mRNA and further suggest a link between the signal-transduction pathway and alternative splicing.

Synergistic antiproliferative and apoptotic effects induced by mixed epidermal growth factor receptor inhibitor ZD1839 and nitric oxide donor in human prostatic cancer cell lines

BACKGROUND

The specific inhibitor of epidermal growth factor receptor (EGFR) tyrosine kinase, ZD1839 induces potent antitumoral effects on several advanced cancer types. The present study was undertaken to determine whether the combination of ZD1839 with an agent donating nitric oxide (NO(*)), sodium nitroprusside (SNP) results in a synergy of anticarcinogenic responses on metastatic prostate cancer (PC) cells.

METHODS

The antiproliferative and apoptotic/necrotic effects of ZD1839 and SNP alone or in combination were estimated on EGF- and serum-stimulated LNCaP, DU145, and PC3 cells by MTT growth tests, trypan blue dye exclusion method, and flow cytometric analyses. Moreover, the cellular ceramide levels were evaluated by the diacylglycerol kinase enzymatic method and the amounts of cytosolic cytochrome c by ELISA assays.

RESULTS

ZD1839 and SNP alone or in combination at lower concentrations induced an inhibition of EGF- and serum-stimulated growth of LNCaP, DU145, and PC3 concomitant with an arrest in the G1 phase of cellular cycle. Interestingly, the mixed ZD1839 and SNP also caused a more substantial apoptotic/necrotic death of these PC cells as compared to drugs alone. Moreover, we have observed that an inhibition of acidic sphingomyelinase, hydrogen peroxide (H(2)O(2)) accumulation and caspase cascades results in a significant reduction of apoptotic/necrotic death induced by mixed ZD1839 and SNP in EGF-stimulated PC3 cells. In addition, the combined ZD1839 plus SNP also induced a higher cellular ceramide and reactive oxygen species (ROS) production, mitochondrial transmembrane potential decrease, and cytochrome c amount released into cytosol as compared to drugs alone.

CONCLUSIONS

The simultaneous use of EGFR inhibitor and compound releasing NO(*) might lead to a synergy in the ceramide and ROS production which might cause cellular membrane damages resulting in a massive apoptotic/necrotic death of metastatic PC cells.

The present and future for gene and viral therapy of directly accessible prostate and squamous cell cancers of the head and neck

Gene therapy has been in a continuous evolutionary process since the first approved trial occurred in 1990 at the National Institute of Health. In the USA, as of March 2004, there were 619 approved gene therapy/transfer protocols and 405 of these were for cancer treatment. Another 294 trials are in progress worldwide, with most concentrated in Europe. However, cancer gene therapy is in its relative infancy when compared with the well-established use of chemo-radiotherapy for treating cancer. As the field develops it is becoming clear that using gene therapy in conjunction with established chemo-radiotherapy approaches is yielding the best results. This concept shall be reviewed in the context of the status of the field, and a future direction based on a combination of gene therapy with small molecule modification of sphingolipid metabolism shall be discussed.

Dissection of the multiple mechanisms of TNF-alpha-induced apoptosis in liver injury

Tumor necrosis factor (TNF)-alpha-induced hepatocyte apoptosis is implicated in a wide range of liver diseases including viral hepatitis, alcoholic hepatitis, ischemia/reperfusion liver injury, and fulminant hepatic failure. TNF-alpha exerts a variety of effects that are mediated mainly by TNF-receptor 1 (TNF-R1) in cell death. The activation of TNF-R1 leads to the activation of multiple apoptotic pathways involving the activation of the pro-death Bcl-2 family proteins, reactive oxygen species, C-Jun NH2-terminal kinase, cathepsin B, acidic sphingomyelinase and neutral sphingomyelinase. These pathways are closely interlinked and mainly act on mitochondria, which release the apoptogenic factors and other events, resulting in apoptosis. This article reviews the recent progress in the molecular mechanisms of TNF-alpha-induced apoptosis in hepatocytes, and discusses how these molecular findings are shaping our understanding of the pathogenesis of liver diseases and our strategy to develop novel therapeutics.

p53-Independent ceramide formation in human glioma cells during gamma-radiation-induced apoptosis

Although the p53 tumor-suppressor gene product plays a critical role in apoptotic cell death induced by DNA-damaging chemotherapeutic agents, human glioma cells with functional p53 were more resistant to gamma-radiation than those with mutant p53. U-87 MG cells with wild-type p53 were resistant to gamma-radiation. U87-W E6 cells that lost functional p53, by the expression of type 16 human papillomavirus E6 oncoprotein, became susceptible to radiation-induced apoptosis. The formation of ceramide by acid sphingomyelinase (A-SMase), but not by neutral sphingomyelinase, was associated with p53-independent apoptosis. SR33557 (2-isopropyl-1-(4-[3-N-methyl-N-(3,4-dimethoxybphenethyl)amino]propyloxy)benzene-sulfonyl) indolizine, an inhibitor of A-SMase, suppressed radiation-induced apoptotic cell death. In contrast, radiation-induced A-SMase activation was blocked in glioma cells with endogenous functional p53. The expression of acid ceramidase was induced by gamma-radiation, and was more evident in cells with functional p53. N-oleoylethanolamine, which is known to inhibit ceramidase activity, unexpectedly downregulated acid ceramidase and accelerated radiation-induced apoptosis in U87-W E6 cells. Moreover, cells with functional p53 could be sensitized to gamma-radiation by N-oleoylethanolamine, which suppressed radiation-induced acid ceramidase expression and then enhanced ceramide formation. Sensitization to gamma-radiation was also observed in U87-MG cells depleted of functional p53 by retroviral expression of small interfering RNA. These results indicate that ceramide may function as a mediator of p53-independent apoptosis in human glioma cells in response to gamma-radiation, and suggest that p53-dependent expression of acid ceramidase and blockage of A-SMase activation play pivotal roles in protection from gamma-radiation of cells with endogenous functional p53.

Implications of galactocerebrosidase and galactosylcerebroside metabolism in cancer cells

Galactosylcerebroside is known to be overexpressed upon the cellular surface of a variety of cancers. In squamous cell carcinomas of the head and neck, one explanation for galactosylcerebroside accumulation has been identified as a transcriptional repression of the galactocerebrosidase gene. Galactocerebrosidase is the enzyme responsible for degrading galactosylcerebroside to ceramide. Ceramide is an important apoptosis activator, whereas galactosylcerebroside functions as an inhibitor. A shift of the ceramide metabolism balance in favor of glycosylated forms has been identified as a mechanism of drug resistance for several antineoplastic agents. Our review elaborates on possible explanations for galactocerebrosidase suppression and on other explanations for increased glycosphingolipid concentration within cancer cell membranes. Furthermore, conjecturable influences of a repressed galactocerebrosidase expression on tumor biology are to be explained. The inhibiting transcription factors YY1 and AP2 have been identified as potential galactocerebrosidase gene suppressors. The resulting accumulation of galactosylcerebroside promotes a reduction of cellular adhesion and inhibits apoptosis, leading to increased cellular growth, migration and prolonged cell survival contributing to carcinogenesis.

Golgi fragmentation is associated with ceramide-induced cellular effects

Ceramide has been shown to cause anoikis, a subtype of apoptosis due to inadequate cell adhesion. However, the underlying mechanism is unclear. Herein, we report that D-e-C6-ceramide (D-e-Cer), via generating sphingosine, disrupts the Golgi complex (GC), which is associated with various cellular effects, including anoikis. Treatment of HeLa cells with D-e-Cer caused cell elongation, spreading inhibition, rounding, and detachment before apoptosis (anoikis). In D-e-Cer-treated cells, glycosylation of beta1 integrin in the GC was inhibited, thus its associated integrin receptors failed to translocate to the cell surface. Ceramide treatment also inhibited the reorganization of both microtubule and F-actin cytoskeletons, focal adhesions, and filopodia. These cellular effects were preceded by fragmentation of the Golgi complex. In contrast, L-e-C6-ceramide (L-e-Cer), the enantiomer of D-e-Cer, failed to induce these cellular effects. Mass spectrometric analysis revealed that treatment HeLa cells with D-e-Cer but not L-e-Cer caused a >50-fold increase in the levels of sphingosine, a product of hydrolysis of ceramide. Treatment with D-e-sphingosine and its enantiomer, L-e-sphingosine, caused massive perinuclear vacuolization, Golgi fragmentation, and cell rounding. Together, these results suggest that sphingosine generated from hydrolysis of ceramide causes the GC disruption, leading to various cellular effects.

Sphingosine kinase activity counteracts ceramide-mediated cell death in human melanoma cells: role of Bcl-2 expression

While most of the pharmacological therapies for melanoma utilize the apoptotic machinery of the cells, the available therapeutic options are limited due to the ability of melanoma cells to resist programmed cell death. Human melanoma cell lines A-375 and M186 are sensitive to ceramide- and Fas-induced cell death, while Mel-2a and M221 are resistant. We have now found that Mel-2a and M221 cells have a significantly higher ceramide/sphingosine-1-phosphate (S1P) ratio than A-375 and M186 cells. As sphingosine kinase (SphK) type 1 plays a critical role in determining the dynamic balance between the proapoptotic sphingolipid metabolite ceramide and the prosurvival S1P, we examined its role in apoptosis of melanoma cells. Increasing SphK1 expression reduced the sensitivity of A-375 melanoma cells to Fas- and ceramide-mediated apoptosis. Conversely, downregulation of SphK1 with small interfering RNA decreased the resistance of Mel-2a cells to apoptosis. Importantly, overexpression of the prosurvival protein Bcl-2 in A-375 cells markedly stimulated SphK1 expression and activity, while downregulation of Bcl-2 reduced SphK1 expression. This link between Bcl-2 and SphK1 might be an additional clue to chemotherapy resistance of malignant melanoma.

Mechanisms of platelet-activating factor (PAF)-mediated responses in the lung

Platelet-activating factor (PAF) is a potent lipid mediator that has been implicated in asthma, sepsis, acute lung injury and ischemia/reperfusion injury. Its actions in the lungs include vasoconstriction, bronchoconstriction, and edema formation. Despite the fact that PAF exerts these actions within minutes, they are mediated by other lipid mediators, in particular eicosanoids generated by cyclooxygenase and lipoxygenase enzymes and sphingolipids generated by acid sphingomyelinase.We will discuss the mechanisms of the PAF-induced pressor responses that are triggered by thromboxane A(2) and leukotrienes, as well the PAF-induced increase in vascular permeability that is mediated by prostaglandin E(2) (PGE(2)) and ceramide.

Effects of Acid Sphingomyelinase Deficiency on Male Germ Cell Development and Programmed Cell Death1

Deficiency of acid sphingomyelinase (ASM), an enzyme responsible for producing a pro-apoptotic second messenger ceramide, has previously been shown to promote the survival of fetal mouse oocytes in vivo and to protect oocytes from chemotherapy-induced apoptosis in vitro. Here we investigated the effects of ASM deficiency on testicular germ cell development and on the ability of germ cells to undergo apoptosis. At the age of 20 weeks, ASM knock-out (ASMKO) sperm concentrations were comparable with wild-type (WT) sperm concentrations, whereas sperm motility was seriously affected. ASMKO testes contained significantly elevated levels of sphingomyelin at the age of 8 weeks as detected by high-performance, thin-layer chromatography. Electron microscopy revealed that the testes started to accumulate pathological vesicles in Sertoli cells and in the interstitium at the age of 21 days. Irradiation of WT and ASMKO mice did not elevate intratesticular ceramide levels at 16 h after irradiation. In situ end labeling of apoptotic cells also showed a similar degree of cell death in both groups. After a 21-day recovery period, the numbers of primary spermatocytes and spermatogonia at G2 as well as spermatids were essentially the same in the WT and ASMKO testes, as detected by flow cytometry. In serum-free cultures both ASMKO and WT germ cells showed a significant increase in the level of ceramide, as well as massive apoptosis. In conclusion, ASM is required for maintenance of normal sphingomyelin levels in the testis and for normal sperm motility, but not for testicular ceramide production or for the ability of the germ cells to undergo apoptosis.

Anticancer compounds and sphingolipid metabolism in the colon

Sphingomyelin metabolism generates anticancer signals such as ceramide and sphingosine that may inhibit cell proliferation, and induce differentiation and apoptosis. Changes of sphingomyelin metabolism are found to be associated with tumorigenesis in various tissues and a particular link between sphingomyelin metabolism and colon cancer has been indicated. The effects of several anticancer drugs on sphingomyelin metabolism have been examined recently and there is an increasing interest in discovering new drugs taking sphingomyelin as a target. The present review outlines the sphingomyelin metabolism pathway, introduces the evidence linking sphingomyelin to colon cancer, and summarizes the anticancer drugs and dietary factors that affect the metabolism of sphingomyelin and, thus, the production of the anticancer messengers in the colon.

NAD+/NADH and/or CoQ/CoQH2 ratios from plasma membrane electron transport may determine ceramide and sphingosine-1-phosphate levels accompanying G1 arrest and apoptosis

To elucidate possible biochemical links between growth arrest from antiproliferative chemotherapeutic agents and apoptosis, our work has focused on agents (EGCg, capsaicin, cis platinum, adriamycin, anti-tumor sulfonylureas, phenoxodiol) that target tNOX. tNOX is a cancer-specific cell surface NADH oxidase (ECTO-NOX protein), that functions in cancer cells as the terminal oxidase for plasma membrane electron transport. When tNOX is active, coenzyme Q(10) (ubiquinone) of the plasma membrane is oxidized and NADH is oxidized at the cytosolic surface of the plasma membrane. However, when tNOX is inhibited and plasma membrane electron transport is diminished, both reduced coenzyme Q(10) (ubiquinol) and NADH would be expected to accumulate. To relate inhibition of plasma membrane redox to increased ceramide levels and arrest of cell proliferation in G(1) and apoptosis, we show that neutral sphingomyelinase, a major contributor to plasma membrane ceramide, is inhibited by reduced glutathione and ubiquinone. Ubiquinol is without effect or stimulates. In contrast, sphingosine kinase, which generates anti-apoptotic sphingosine-1-phosphate, is stimulated by ubiquinone but inhibited by ubiquinol and NADH. Thus, the quinone and pyridine nucleotide products of plasma membrane redox, ubiquinone and ubiquinol, as well as NAD(+) and NADH, may directly modulate in a reciprocal manner two key plasma membrane enzymes, sphingomyelinase and sphingosine kinase, potentially leading to G(1) arrest (increase in ceramide) and apoptosis (loss of sphingosine-1-phosphate). As such, the findings provide potential links between coenzyme Q(10)-mediated plasma membrane electron transport and the anticancer action of several clinically-relevant anticancer agents.

P-glycoprotein is implicated in the inhibition of ceramide-induced apoptosis in TF-1 acute myeloid leukemia cells by modulation of the glucosylceramide synthase pathway

OBJECTIVE

Ceramide, an intermediate of apoptosis induction in response to chemotherapy, can be detoxified by glycosylation at the cytoplasmic surface of the Golgi membrane. P-glycoprotein (p-gp) might augment ceramide glycosylation by translocating glucosylceramide (GC) across the Golgi membrane. We aimed to show that glucosylceramide synthase (GCS) activity is linked to p-gp expression and resistance to ceramide-induced apoptosis in acute myeloid leukemia (AML).

METHODS

Apoptosis and cell-cycle analysis were measured using propidium iodide staining and flow cytometry. Fluorescent microscopy assessed p-gp expression in, and rhodamine 123 uptake by, the Golgi. P-gp interaction with GC was assessed by modulation of rhodamine accumulation. The GCS activity assay was based upon the transfer of UDP-(3)H-glucose to C8-ceramide to form radiolabeled GC, by rate-limiting cell-derived GCS. TLC and fluorimetry were used to measure the metabolites of fluorescent ceramide. Cell viability was measured using 7-amino-actinomycin D staining and flow cytometry with an internal standard for cell enumeration.

RESULTS

P-gp(+) cell lines (KG1a, TF-1) were resistant to C8-ceramide-induced apoptosis compared to p-gp(-) cell lines (HL-60, U937). P-gp inhibitors GF120918 and cyclosporin A enhanced ceramide-induced apoptosis in the p-gp expressing cells. P-gp expression was identified in the Golgi of these cells. Pgp's efflux function in TF-1 but not KG1a cells was inhibited by glucosylceramide. In the presence of p-gp inhibitors, R123 accumulation in the Golgi of TF-1 cells was lost, and GCS activity and lactosylceramide formation were downregulated. Intact cells were necessary for the involvement of p-gp in the regulation of GCS activity.

CONCLUSION

Our data suggests that ceramide induces apoptosis in AML cells and that p-gp confers resistance to ceramide-induced apoptosis, with modulation of the ceramide-glucosylceramide pathway making a marked contribution to this resistance in TF-1 cells.

Differential regulation of ceramide in lipid-rich microdomains (rafts): Antagonistic role of palmitoyl:protein thioesterase and neutral sphingomyelinase 2

Cell differentiation and myelination involve a fine balance between stasis and programmed cell death, yet the genes that regulate this have not been clearly defined. We therefore studied two key gene products involved in oligodendrocyte plasma membrane lipid metabolism and their antagonistic role in ceramide-mediated cell death signaling. Overexpression of palmitoyl:protein thioesterase (PPT1; verified by Western blot of the V5-tagged protein and increased enzyme activity) resulted in decreased ceramide in the detergent-resistant microdomain (DRM, or raft) relative to cholesterol and sphingomyelin (SM). This PPT1 overexpression also resulted in protection against cell death induced by either staurosporine or C(2)-ceramide. In contrast, overexpression of neutral sphingomyelinase 2 (NSMase2; verified by Western blot of the FLAG-tagged protein and increased enzyme activity) resulted in increased membrane NSMase and increased ceramide in rafts relative to cholesterol and SM. The difference in SM and ceramide turnover was quantitated by [(3)H]palmitate pulse-chase labeling. Furthermore, when NBD-SM was added to cells, it was hydrolyzed by NSMase-transfected cells at more than twofold the rate in untransfected cells. NSMase2 overexpression enhanced cell death induced by staurosporine or C(2)-ceramide, in contrast to the protective effect of PPT1 overexpression. The presence of a fraction of both PPT1 and NSMase2 in rafts together with their substrates (palmitoylated proteins and SM, respectively) suggests a mechanism for dynamic palmitoylation/depalmitoylation of certain proteins in controlling cell death via NSMase activation.

Selective Inhibition of Juxtanuclear Translocation of Protein Kinase C βII by a Negative Feedback Mechanism Involving Ceramide Formed from the Salvage Pathway

In a previous study, we showed that protein kinase C betaII (PKC betaII) translocated to a novel juxtanuclear compartment as observed in several cell types (Becker, K. P., and Hannun, Y. A. (2003) J. Biol. Chem. 278, 52747-52754). In this study, we noted the absence of this translocation in MCF-7 breast cancer cells, and we examined the mechanisms underlying this selectivity of response. We show that sustained stimulation of PKC betaII with 4beta-phorbol 12-myristate 13-acetate (PMA) resulted in accumulation of ceramide in MCF-7 cells but not in those cells that showed juxtanuclear translocation of PKC betaII. Addition of exogenous ceramides or formation of endogenous ceramide by the action of bacterial sphingomyelinase prevented PMA-induced translocation of PKC betaII in HEK 293 cells. On the other hand, inhibition of ceramide accumulation with fumonisin B1 restored the ability of PMA to induce translocation of PKC betaII in MCF-7 cells. Taken together, the results showed that endogenous ceramide is both necessary and sufficient for preventing juxtanuclear translocation of PKC betaII in response to PMA. Investigation of the mechanisms of ceramide generation in response to PMA revealed that PMA activated the salvage pathway of ceramide formation and not the de novo pathway. This conclusion was based on the following: 1) the ability of fumonisin B1 but not myriocin to inhibit ceramide formation, 2) the ability of PMA to induce increases in palmitate-labeled ceramide only under chase labeling but not acute pulse labeling, 3) the induction of the levels of sphingosine but not dihydrosphingosine in response to PMA, and 4) induction of sphingomyelin hydrolysis in response to PMA. Together, these results define a novel pathway of regulated formation of ceramide, the salvage pathway, and they define a role for this pathway in regulating juxtanuclear translocation of PKC betaII.

Life and death decisions: ceramide generation and EGF receptor trafficking are modulated by oxidative stress

Reactive oxidants are associated with the pathogenesis of pulmonary diseases and affect various cell functions, from proliferation to apoptosis. We have shown that oxidants exert growth control on airway epithelial cells by modulating upstream receptor function. Additionally, hydrogen peroxide-mediated oxidative stress modulates ceramide levels to induce apoptosis in lung epithelium. Depletion of glutathione in lung epithelial cells results in ceramide accumulation, suggesting that ceramide elevation, coupled to oxidative stress, initiates apoptosis. While it is desirable to prevent cell death and tissue injury induced by oxidants in diseases such as asthma or acute respiratory distress syndrome, the opposite is sought in cancer. But oxidants may also activate growth factor receptors, enhancing cell proliferation and facilitating tumor promotion. Under oxidative stress, phosphorylation of the epidermal growth factor receptor (EGFR) is abrogated at tyrosine 1,045, the docking site for the ubiquitin ligase c-Cbl, rendering EGFR unable to recruit c-Cbl and be ubiquitylated and degraded. We thus proposed that this deficiency, which confers prolonged receptor signaling at the plasma membrane, links oxidative stress, EGFR, and tumorigenesis. Decoding the molecular interactions between oxidative stress and ceramide pathways and characterizing ubiquitylation control of receptor desensitization should provide new strategies for intervention in diverse pulmonary diseases and in diagnosing and eradicating cancer.

Enzymes of Sphingolipid metabolism in Drosophila melanogaster

Sphingolipids are important structural components of membranes that delimit the boundaries of cellular compartments, cells and organisms. They play an equally important role as second messengers, and transduce signals across or within the compartments they define to initiate physiological changes during development, differentiation and a host of other cellular events. For well over a century Drosophila melanogaster has served as a useful model organism to understand some of the fundamental tenets of development, differentiation and signaling in eukaryotic organisms. Directed approaches to study sphingolipid biology in Drosophila have been initiated only recently. Nevertheless, earlier phenotypic studies conducted on genes of unknown biochemical function have recently been recognized as mutants of enzymes of sphingolipid metabolism. Genome sequencing and annotation have aided the identification of homologs of recently discovered genes. Here we present an overview of studies on enzymes of the de novo sphingolipid biosynthetic pathway, known mutants and their phenotypic characterization in Drosophila.

Signal transduction pathways regulating cyclooxygenase-2 in lipopolysaccharide-activated primary rat microglia

Microglia are the major cell type involved in neuroinflammatory events in brain diseases such as encephalitis, stroke, and neurodegenerative disorders, and contribute significantly to the release of prostaglandins (PGs) during neuronal insults. In this report, we studied the immediate-early intracellular signalling pathways in microglia, following bacterial lipopolysaccharide (LPS) stimulation, leading to the synthesis and release of PGE2. Here we show that LPS induces cyclooxygenase (COX) 2 by activating sphingomyelinases leading to the release of ceramides, which in turn, activate the p38 mitogen-activated protein kinases (MAPK), but not the p42/44 MAPK. We further show that exogenously added ceramide analogue (C2-ceramide) also induce PGE2 synthesis through a p38 MAPK-dependent pathway. This potential nature of ceramides in activating microglia suggests that endogenously produced ceramides during neuronal apoptosis in ischemia or neurodegenerative diseases could also contribute to the amplification of neuroinflammatory events. In contrast to protein kinase C (PKC) and phosphocholine-specific phospholipase C (PC-PLC), which transcriptionally regulate LPS-induced COX-2 synthesis, inhibition of phospholipase A2 (PLA2) has no effect on COX-2 transcription, although it inhibits the release of PGE2. Transcriptional regulation of LPS-induced COX-2 by PKC is further proved by the ability of the PKC inhibitor, Gö 6976, to inhibit LPS-induced 8-isoprostane synthesis, but not affecting LPS-induced COX-2 activity. Our data with 8-isoprostane also indicates that COX-2 plays a major role in ROS production in LPS-activated microglia. This detailed view of the intracellular signaling pathway in microglial activation and COX-2 expression opens a new therapeutic window in the search for new and more effective central anti-inflammatory agents.