Induction of apoptotic DNA fragmentation and cell death by natural ceramide

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

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

Killing tumours by ceramide-induced apoptosis: a critique of available drugs

Over 1000 research papers have described the production of programmed cell death (apoptosis) by interventions that elevate the cell content of ceramide (Cer). Other interventions, which lower cellular Cer, have been found to interfere with apoptosis induced by other agents. Some studies have shown that slowing the formation of proliferation-stimulating sphingolipids also induces apoptosis. These relationships are due to the two different aspects of Cer: Cer itself produces apoptosis, but metabolic conversion of Cer into either sphingosine 1-phosphate or glucosphingolipids leads to cell proliferation. The balance between these two aspects is missing in cancer cells, and yet intervention by stimulating or blocking only one or two of the pathways in Cer metabolism is very likely to fail. This results from two properties of cancer cells: their high mutation rate and the preferential survival of the most malignant cells. Tumours treated with only one or two drugs that elevate Cer can adjust the uncontrolled processes to either maintain or to 'aggravate' the excessive growth, angiogenesis and metastasis characteristics of tumours. These treatments might simply elevate the production of growth factors, receptors and other substances that reduce the effectiveness of Cer. Tumour cells that do not adapt in this way undergo apoptosis, leaving the adapted cells free to grow and, ultimately, to 'subdue' their host. Thus it is important to kill every type of cancer cell present in the tumour rapidly and simultaneously, using as many different agents to control as many pathways as possible. To aid this approach, this article catalogues many of the drugs that act on different aspects of Cer metabolism. The techniques described here may lead to the development of practical chemotherapy for cancer and other diseases of excess proliferation.

Ceramide increases mitochondrial free calcium levels via caspase 8 and Bid: role in initiation of cell death

We investigated how the mitochondrial phase of ceramide-mediated cell death is initiated in nerve growth factor (NGF)-differentiated PC12 cells. We distinguished three independent effects of ceramide: free radical production; a transient increase in cytosolic free calcium; and a long-lasting increase in mitochondrial free calcium. Only the latter led to cell death, which could be prevented by buffering of mitochondrial calcium with the calcium binding protein calbindin D-28K ectopically expressed in mitochondria. We showed that mitochondrial calcium did not increase as a result of the increase in cytosolic free calcium levels. Rather, it appears to derive from the endoplasmic reticulum (ER) since dantrolene, which inhibits release of calcium from ER into cytosol through ryanodine receptors, prevented the increase in cytosolic free calcium but potentiated the increase in mitochondrial free calcium. This suggests that a transfer of calcium occurs directly, or very locally, between the two organelles. This transfer implicated activation of caspase 8 and cleavage of its substrate Bid, a previously unknown function of these cell death intermediaries. The increase in mitochondrial free calcium was also responsible for the release of cytochrome c into the cytosol, underlining the critical role it plays in ceramide-mediated cell death.

New advances on structural and biological functions of ceramide in apoptotic/necrotic cell death and cancer

Recent data on the cellular ceramide functions and its involvement in the apoptotic/necrotic cell death as well as its anticarcinogenic properties are presented. The emphasis is on the connections between the ceramide and caspase signaling pathways during the apoptotic cell death process. Notably, the experimental strategies and pharmacological tools used for establishment of the role of ceramide in triggering cell death are described. Moreover, the importance of a compartmentation of endogenous ceramide within the plasma membrane microdomains, lysosomes and mitochondria is discussed. Information on the deregulated functions of ceramide and caspase signaling pathways in several metastatic cancer types is also presented.

Subcellular distribution and metabolic fate of exogenous ceramides taken up by HL-60 cells

Ceramides (Cer) are key intermediates in the metabolism of sphingomyelin and are also important second messengers. We report that natural long-chain ceramides added to the incubation medium in microgram amounts are internalized in HL-60 cells as well as the short-chain analogue C2-Cer and targeted to various subcellular compartments. No significant difference was detected in the ability of HL-60 cells to metabolize exogenous Cer containing a short (acetyl) versus long (palmitoyl or oleoyl) acyl chain. After a 2-h incubation time with [14C]-C16 ceramides, most of the cell-bound radioactivity was found in free ceramides. Sphingomyelin was the major metabolized sphingolipid containing labeled ceramides and only a small proportion of exogenous ceramides were converted to neutral glycolipids and gangliosides. Up to 20% of the exogenous ceramides taken up by the cells were recovered in mitochondria, mostly as authentic C16 ceramides and C16 sphingomyelin, along with a trace amount of labeled GM3 ganglioside. These results are consistent with the notion that exogenous natural ceramides enter cells, can be further metabolized in situ and partly targeted to mitochondria, which are known to be involved in the control of programmed cell death.

Ceramide in apoptosis: a revisited role

The sphingolipid ceramide has recently emerged as a new transducer or modulator of apoptotic cell death. This function, however, has recently been challenged. Here, in the light of recent observations, the role of ceramide in apoptosis signaling is discussed.

Sphingosine-induced c-jun expression: differences between sphingosine- and C2-ceramide-mediated signaling pathways

Sphingolipids such as ceramide and sphingosine are putative intracellular signal mediators in cell differentiation, growth inhibition and apoptosis. Previously, we reported that C2-ceramide induced c-jun expression in apoptosis of human leukemia HL-60 cells. Here we report that sphingosine also induced c-jun expression in apoptosis of HL-60 cells. Sphingosine-induced c-jun expression was stimulated by H-89, a protein kinase A inhibitor, whereas C2-ceramide-induced c-jun expression was inhibited by protein kinase C inhibitors. Furthermore, H-89 potentiated sphingosine-induced but not C2-ceramide-induced growth inhibition. These results suggest that sphingosine and C2-ceramide might induce c-jun expression and apoptosis in distinct signaling pathways.

Disruption of mitochondria is an early event during dolichyl monophosphate-induced apoptosis in U937 cells

Dolichyl monophosphate (Dol-P) is involved in the attachment of carbohydrate chains to proteins in the formation of N-linked glycoprotein. We found that this compound induces apoptosis in human leukemia U937 cells. During this apoptotic execution, the increase of plasma membrane fluidity (5-20 min), reduction in mitochondrial transmembrane potential (delta psi m) and translocation of apoptosis-inducing factor (1-3 hr), caspase-3-like protease activation (2-4 hr), chromatin condensation and DNA ladder formation (3-4 hr) were observed successively. In this study, we examined mitochondrial morphological changes by electron microscopy and delta psi m by JC-1 from immediately after treatment of Dol-P. After 5 min of treatment, we observed clearly that mitochondrial cristae began to be disrupted ultrastructurally and almost all the cristae were disintegrated after 1 hr of treatment. The delta psi m of Dol-P treated cells was reduced to 34% as compared with that of control cells immediately after treatment and was quartered within 1 hr. The reduction in delta psi m was not inhibited by cyclosporin A, N-acetyl-L-cysteine and vitamin E. These results indicate that mitochondrial disruption is one of the first triggering events of Dol-P-induced apoptosis.

Activation of the JAK/STAT pathway by ceramide in cultured human fibroblasts

Endogenous ceramide (CER) was generated by treatment of cultured fibroblasts with sphingomyelinase (SMase) from Bacillus cereus. A 30 min treatment with 0.1-0.3 U/ml SMase induced a dose-dependent increase in the intracellular level of CER. The activation of the transcription factors signal transducer and activator of transcription (STAT) 1 and STAT3 by SMase was investigated by determination of the phosphorylation state by immunoblot, and of DNA binding activity by electrophoretic mobility shift assay. SMase treatment induced a dose-dependent Tyr-phosphorylation of STAT1/3. SMase also enhanced STAT1/3 DNA binding activity in a dose-dependent manner. Concomitantly, SMase enhanced the Tyr-phosphorylation of Janus kinase (JAK) 2, a Tyr-kinase localized upstream of STATs in the JAK/STAT pathway. The Tyr-kinase inhibitor genistein and the JAK inhibitor AG490 both prevented JAK2 Tyr-phosphorylation, together with STAT1 and STAT3 Tyr-phosphorylation and binding activity. The SMase-induced increase in STAT1/3 binding activity was prevented by methyl-beta-cyclodextrin, a cholesterol binding agent that causes a loss of compartmentalization of the molecules located in caveolae. This increase was also prevented by the MEK inhibitor PD98059, thus demonstrating the role of the MEK/ERK pathway in this system. Besides ERK, SMase activated other signaling kinases such as JNK and p38. Exogenous natural CER also activated STAT1/3 binding activity, which indicates that most probably, endogenous CER is the second messenger involved in the effect of SMase. These results describe a crosstalk between the SMase/CER and the JAK/STAT signaling pathways and include JAK2 within the range of CER-activated intracellular kinases.

Nitric oxide production in living neurons is modulated by sphingosine: a fluorescence microscopy study

An investigation was carried out into the possible effect of sphingosine (Sph) on nitric oxide (NO) production in living neurons. Differentiated granule cells were used in a dynamic videoimaging analysis of single cells labeled, simultaneously, with FURA-2 and the NO indicator 4,5-diaminofluorescein. The results demonstrate that Sph exerts a potent inhibitory effect on the Ca2+-dependent production of NO, without modifying the [Ca2+]i. The effect appears to be specific as neither ceramide nor Sph-1-phosphate had any effect on the NO and [Ca2+]i levels. The data demonstrate that Ca2+-dependent NO production is a specific Sph target in living granule cells, suggesting that this bioactive sphingoid plays a relevant role in neuronal NO signaling.

A role of the mitochondrial apoptosis-inducing factor in granulysin-induced apoptosis

Granulysin is a cytolytic molecule released by CTL via granule-mediated exocytosis. In a previous study we showed that granulysin induced apoptosis using both caspase- and ceramide-dependent and -independent pathways. In the present study we further characterize the biochemical mechanism for granulysin-induced apoptosis of tumor cells. Granulysin-induced death is significantly inhibited by Bcl-2 overexpression and is associated with a rapid (1-5 h) loss of mitochondrial membrane potential, which is not mediated by ceramide generation and is not inhibited by the general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. Ceramide generation induced by granulysin is a slow event, only observable at longer incubation times (12 h). Apoptosis induced by exogenous natural (C(18)) ceramide is truly associated with mitochondrial membrane potential loss, but contrary to granulysin, this event is inhibited by benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. Ceramide-induced apoptosis is also completely prevented by Bcl-2 overexpression. The nuclear morphology of cells dying after granulysin treatment in the presence of caspase inhibitors suggested the involvement of mitochondrial apoptosis-inducing factor (AIF) in granulysin-induced cell death. We demonstrate using confocal microscopy that AIF is translocated from mitochondria to the nucleus during granulysin-induced apoptosis. The majority of Bcl-2 transfectants are protected from granulysin-induced cell death, mitochondrial membrane potential loss, and AIF translocation, while a small percentage are not protected. In this small percentage the typical nuclear apoptotic morphology is delayed, being of the AIF type at 5 h time, while at longer times (12 h) the normal apoptotic morphology is predominant. These and previous results support a key role for the mitochondrial pathway of apoptosis, and especially for AIF, during granulysin-induced tumoral cell death.

Apoptotic death by ceramide: will the real killer please stand up?

Many studies have shown that the addition of ceramide to an incubation medium, or procedures that lead to increased ceramide concentrations, can begin a process that leads to slowing of cell growth or apoptotic cell death. Only a few studies have examined the nature of the accumulating ceramide: is it composed of a fatty acid and sphinganine, or a fatty acid and sphingosine? Of the studies involving addition of ceramide to a cell culture, almost all have found that the sphingosine amide is active, not the sphinganine amide. Nearly all of these studies have utilized the rare form of ceramide, containing an acetyl group rather than the commonly found palmitoyl, stearoyl, or longer group. Acetyl sphingosine produces some unexpected effects with cells, the most striking being the formation of reactive oxygen species and mitochondrial damage. This mitochondrial damage appears to be an essential step in apoptosis. The possibility should be considered that the reactive oxygen species appear as the result of oxidation of the allylic alcohol group in unsaturated ceramides. This question is very relevant to a host of ceramide functions, particularly cancer chemotherapy and cell growth.

Modulation of ceramide-induced NF-kappaB binding activity and apoptotic response by caffeic acid in U937 cells: comparison with other antioxidants

Ceramide acts as second messenger in the signal transduction triggered by a variety of stress stimuli and extracellular agents. Stress response through ceramide is involved in the development of many human diseases, such as atherosclerosis, inflammation, neurodegenerative disorders, and acquired immunodeficiency syndrome. Dietary polyphenols have been reported to exert a beneficial effect on the onset and development of most of these human chronic-degenerative pathologies. However, the mechanisms underlying this beneficial effect are mostly not understood at the present. To investigate the ability of polyphenols in modulating fundamental cellular functions, we studied the effect of caffeic acid, a widespread phenolic acid largely present in human diet, in the modulation of ceramide-induced signal transduction pathway leading to apoptosis in U937 cells, in comparison with other established antioxidants of nutritional interest (N-acetylcysteine, d-alpha-tocopherol acetate and ascorbic acid). Our results indicate that caffeic acid efficiently inhibits both ceramide-induced NF-kappaB binding activity and apoptosis at micromolar concentration. Other antioxidants tested are totally ineffective in inhibiting apoptosis, although affecting NF-kappaB activation. Caffeic acid was found to inhibit protein tyrosine kinase activity, suggesting that this mechanism can be on the basis of the inhibition of apoptosis. Our results suggest that dietary caffeic acid might modulate ceramide-induced signal transduction pathway and NF-kappaB activation through either antioxidant and nonantioxidant mechanisms.

Comparison of the metabolism of L-erythro- and L-threo-sphinganines and ceramides in cultured cells and in subcellular fractions

Ceramide (Cer) is a key intermediate in the synthetic and degradative pathways of sphingolipid metabolism, and is also an important second messenger. Natural Cer exists in the D-erythro configuration. Three additional, non-natural stereoisomers exist, but conflicting reports have appeared concerning their metabolism. We now compare the stereospecificity of three enzymes in the sphingolipid biosynthetic pathway, namely dihydroceramide (dihydroCer), sphingomyelin (SM) and glucosylceramide synthases, in subcellular fractions and in cultured cells. The L-erythro enantiomers of sphinganine, dihydroCer and Cer do not act as substrates for any of the three enzymes. In contrast, the diastereoisomer, L-threo-sphinganine, is acylated by dihydroCer synthase, and L-threo-dihydroCer and L-threo-Cer are both metabolized to dihydroSM and SM, respectively, but not to dihydroglucosylceramide and glucosylceramide. No significant difference was detected in the ability of SM synthase to metabolize Cer containing a short (hexanoyl) versus long acyl chain (palmitoyl), demonstrating that short-acyl chain Cers mimic their natural counterparts, at least in the sphingolipid biosynthetic pathway.

Cell autonomous apoptosis defects in acid sphingomyelinase knockout fibroblasts

A body of evidence suggests that stress-induced sphingomyelin hydrolysis to the second messenger ceramide initiates apoptosis in some cells. Although studies using lymphoblasts from Niemann-Pick disease patients or acid sphingomyelinase (ASMase)-deficient mice have provided genetic support for this hypothesis, these models have not been universally accepted as definitive. Here, we show that mouse embryonic fibroblasts (MEFs) prepared from asmase mice manifest cell autonomous defects in apoptosis in response to several stresses. In particular, asmase(-/-) MEFs failed to generate ceramide and were totally resistant to radiation-induced apoptosis but remained sensitive to staurosporine, which did not induce ceramide. asmase(-/-) MEFs were also partially resistant to tumor necrosis factor alpha/ actinomycin D and serum withdrawal. Thus, resistance to apoptosis in asmase(-/-) MEFs was not global but rather stress type specific. Most importantly, the sensitivity to stress could be restored in the asmase(-/-) MEFs by administration of natural ceramide. Overcoming apoptosis resistance by natural ceramide is evidence that it is the lack of ceramide, not ASMase, that determines apoptosis sensitivity. The ability to rescue the apoptotic phenotype without reversing the genotype by the product of the enzymatic deficiency provides proof that ceramide is obligate for apoptosis induction in response to some stresses.

Killing cancer cells by poly-drug elevation of ceramide levels: a hypothesis whose time has come?

Many papers have shown that sphingolipids control the balance in cells between growth and proliferation, and cell death by apoptosis. Sphingosine-1-phosphate (Sph1P) and glucosylceramide (GlcCer) induce proliferation processes, and ceramide (Cer), a metabolic intermediate between the two, induces apoptosis. In cancers, the balance seems to have come undone and it should be possible to kill the cells by enhancing the processes that lead to ceramide accumulation. The two control systems are intertwined, modulated by a variety of agents affecting the activities of the enzymes in Cer-GlcCer-Sph1P interdependence. It is proposed that successful cancer chemotherapy requires the use of many agents to elevate ceramide levels adequately. This review updates current knowledge of sphingolipid metabolism and some of the evidence showing that ceramide plays a causal role in apoptosis induction, as well as a chemotherapeutic agent.

Impaired ceramide signalling in spontaneously hypertensive rat vascular smooth muscle: a possible mechanism for augmented cell proliferation

OBJECTIVES

In hypertension, the vascular wall undergoes morphological changes that alter mechanical responses to vasoactive substances. Ceramide is a recently identified second messenger synthesized in response to cytokines such as tumour necrosis factor alpha (TNF-alpha). It has been previously demonstrated that vascular smooth muscle cells (VSMC) from genetically hypertensive rats proliferate at a higher rate than those of normotensive origin. We tested the hypothesis that the ceramide pathway is impaired in VSMC from spontaneously hypertensive rats (SHR).

DESIGN

VSMC were isolated from aortae of SHR and from Wistar-Kyoto (WKY) rats. Ceramide levels were measured under baseline and agonist-stimulated conditions and cell proliferation was monitored.

METHODS

Cell proliferation was determined by cell counting. Ceramide levels were determined via radioactive labelling, high-performance thin-layer chromatography and phosphorimaging. Relative mRNA levels of neutral sphingomyelinase were determined using semi-quantitative polymerase chain reaction (PCR).

RESULTS

Basal ceramide levels in untreated cells were lower in cells from SHR compared to WKY rats. During chronic treatment with TNF-alpha, ceramide levels increased in WKY rat cells but remained unchanged in cells from SHR. TNF-alpha treatment had an inhibitory effect on WKY rat VSMC proliferation, but stimulated proliferation in cells from SHR. Short-term incubation with TNF-alpha resulted in a greater increase in ceramide in cells from WKY rats than those from SHR. Semiquantitative PCR analysis indicated that neutral sphingomyelinase mRNA may be reduced in SHR VSMC.

CONCLUSIONS

We conclude that ceramide synthesis is impaired in vascular smooth muscle from SHR and may contribute to increased VSMC proliferation in hypertension.

The acid sphingomyelinase inhibitor SR33557 counteracts TNF-alpha-mediated potentiation of IL-1beta-induced NF-kappaB activation in the insulin-producing cell line Rinm5F

Cytokines induce nitric oxide (NO) production and cell death in insulin-producing cells in vitro but the signaling pathways mediating the cytokine effects are not well characterized. The aim of this study was to determine whether sphingomyelinase (SMase) participates in cytokine signaling leading to NF-kappaB activation, iNOS induction and cell death in insulin-producing cells. Acute exposure to IL-1beta or TNF-alpha did not affect SMase activities in rat insulinoma (RINm5F) cells. TNF-alpha activated NF-kappaB in gel shift experiments without inducing iNOS--as assessed by nitrite formation--whereas IL-1beta stimulated both NF-kappaB activation and iNOS induction. Natural ceramide did not activate NF-kappaB or iNOS. However, both ceramide and TNF-alpha potentiated IL-1beta- induced activation of NF-kappaB and iNOS. Moreover, the potentiating effects of TNF-alpha were counteracted by the acid SMase inhibitor SR33557. The combination of IL-1beta and IFN-gamma induced apoptosis in RINm5F cells, which was paralleled by a modest increase in acid SMase, whereas ceramide mainly induced necrosis. It is concluded that cytokine-induced beta-cell signaling is associated with the induction of iNOS but not with enhanced SMase activities. However, TNF-alpha-mediated potentiation of the IL-1beta effect may involve an increased sensitivity to basal acid SMase activity. An increased acid SMase activity may participate in the execution of cytokine-induced beta-cell apoptosis.

Involvement of the ceramide signaling pathway in modulating the differentiated state of porcine thyroid cells

Neutral sphingomyelinase (Smase) is a cell membrane-associated phospholipase that hydrolyzes sphingomyelin to phosphocholine and ceramide, a lipid second messenger involved in cell differentiation and/or apoptosis. We first evidenced that porcine cultured thyroid cells could express neutral Smase activity even if thyrotropin (TStH), an essential hormone in thyroid cell differentiation, was found to induce a 1.7-fold decrease in Smase activity. Triggering the ceramide pathway by exogenous addition of neutral bacterial Smase (0.1 U/mL for 48 h), which transiently increased ceramide level by fourfold, drastically modified thyroid cell morphology. The follicle-like structures generated by TSH were disrupted, and the Smase-induced cell spreading was accompanied by a parallel loss of cell ability to iodinate proteins as well as a decrease of the adenylate cyclase system response. These inhibitory effects have been reproduced using short-chain exogenous ceramide analogs (C2-ceramides). Overall these data showed that ceramides emerged as potential mediators of dedifferentiation in thyroid cells.

Biomodulatory role of ceramide in basic fibroblast growth factor-induced proliferation of cerebellar astrocytes in primary culture

To evaluate the role of ceramide in glial growth, primary cultures of quiescent astrocytes from rat cerebellum were stimulated to proliferate by mitogenic doses of basic fibroblast growth factor (bFGF). Parallel to the bFGF mitogenic effect was a marked, and persistent, decrease in cellular ceramide levels. Both in vitro and in culture metabolic studies have led us to exclude both sphingomyelinase and ceramidase involvement in ceramide level variation. Instead, we found evidence of a functional connection between the decrease in ceramide levels and astrocyte proliferation. In fact, cell growth in bFGF-stimulated astrocytes was inhibited by exogenous ceramide and C2-ceramide, maximal inhibition being obtained at a ceramide concentration of 5-10 microM. Under the same conditions, the dihydroderivatives of ceramides were without effect. Following ceramide treatment, the phosphorylation of the MAP kinase isoforms ERK1/2, key components in bFGF-induced cell proliferation, was examined. The administration of antiproliferative doses of ceramide or C2-ceramide, but not of their dihydroderivatives, resulted in a significant inhibition of ERK1/2 activation. In conclusion, our data indicate that the prompt modulation of ceramide levels by bFGF is an early step associated with the signaling pathways responsible for the mitogenic activity of bFGF in astrocytes.

Mitochondrial free calcium levels (Rhod-2 fluorescence) and ultrastructural alterations in neuronally differentiated PC12 cells during ceramide-dependent cell death

Mitochondrial free calcium levels measured by Rhod-2 fluorescence and ultrastructure were examined during cell death in nerve growth factor (NGF)-differentiated PC12 cells that were 1) exposed to C2-ceramide, 2) deprived of serum to induce endogenous ceramide production, or 3) treated with calcium ionophore A23187. Rhod-2 fluorescence in mitochondria and also in the nucleolus increased to a maximum within 3 hours after C2-ceramide treatment or serum withdrawal. In A23187-treated cells, Rhod-2 fluorescence remained at baseline levels. In all three models, enlargement of the endoplasmic reticulum was the first ultrastructural alteration, followed by mitochondrial shrinkage in ionophore-treated cells, but by mitochondrial swelling in the ceramide-dependent models, in which rupture of the outer mitochondrial membrane and unfolding of the inner membrane were frequently seen. Dihydro-C2-ceramide, which did not cause cell death, had no effect on cellular ultrastructure. NGF, which inhibits ceramide-dependent cell death, prevented the effects of serum deprivation on mitochondrial ultrastructure but not on endoplasmic reticulum morphology or Rhod-2 fluorescence. Nuclear shrinkage with loss of nuclear membrane integrity, characterized by nuclear pores, free or surrounded by electron-dense filaments, was a late event in ceramide-dependent cell death. Chromatin condensation and other morphological features associated with apoptosis were seen in only a few atypical cells. Ceramide-mediated cell death, therefore, did not involve classical apoptosis but was mediated by a reproducible series of events beginning in the endoplasmic reticulum, followed by the mitochondria, and then the nucleus. NGF-dependent cell death inhibition intervenes at the mitochondrial level, not by blocking the increase in Rhod-2 fluorescence but by preventing the ultrastructural changes that follow.

Production of ceramides causes apoptosis during early neural differentiation in vitro

To investigate signal transduction pathways leading to apoptosis during the early phase of neurogenesis, we employed PCC7-Mz1 cells, which cease to proliferate and begin to differentiate into a stable pattern of neurons, astroglial cells, and fibroblasts upon incubation with retinoic acid (RA). As part of lineage determination, a sizable fraction of RA-treated cultures die by apoptosis. Applying natural long-chain C(16)-ceramides as well as membrane-permeable C(2)/C(6)-ceramide analogs caused apoptosis, whereas the biologically nonactive C(2)-dihydroceramide did not. Treating PCC7-Mz1 stem cells with a neutral sphingomyelinase or with the ceramidase inhibitor N-oleoylethanolamine elevated the endogenous ceramide levels and concomitantly induced apoptosis. Addition of RA caused an increase in ceramide levels within 3-5 h, which reached a maximum (up to 3.5-fold of control) between days 1 and 3 of differentiation. Differentiated PCC7-Mz1 cells did not respond with ceramide formation and apoptosis to RA treatment. The acidic sphingomyelinase contributed only weakly and the neutral Mg(2+)-dependent and Mg(2+)-independent sphingomyelinases not at all to the RA-mediated production of ceramides. However, ceramide increase was sensitive to the ceramide synthase inhibitor fumonisin B(1), suggesting a crucial role for the de novo synthesis pathway. Enzymatic assays revealed that ceramide synthase activity remained unaltered, whereas serine palmitoyltransferase (SPT), a key enzyme in ceramide synthesis, was activated approximately 2.5-fold by RA treatment. Activation of SPT seemed to be mediated via a post-translational mechanism because levels of the mRNAs coding for the two SPT subunits were unaffected. Expression of marker proteins shows that ceramide regulates apoptosis, rather than differentiation, during early neural differentiation.

Involvement of apoptosis-inducing factor during dolichyl monophosphate-induced apoptosis in U937 cells

Dolichyl monophosphate (Dol-P) has been found to induce apoptosis in human leukemia U937 cells. During this apoptotic execution, the increase of plasma membrane fluidity (5-20 min), caspase-3-like protease activation (2-4 h), chromatin condensation and DNA ladder formation (3-4 h) were observed successively. Here, we report that reduction in mitochondrial transmembrane potential and translocation of apoptosis-inducing factor (AIF) are early events (1-3 h) in the apoptotic process induced by Dol-P in U937 cells. The AIF was concentrated around nuclei and partly translocated to the nuclei, which was confirmed by immunocytochemistry using specific anti-AIF antibody. Both caspase-8 and caspase-3 inhibitors blocked only DNA fragmentation but not mitochondrial processes, AIF migration and chromatin condensation. These results indicate that mitochondrial changes are an early step in the apoptosis induced by Dol-P and AIF is one of the important factors which induce chromatin condensation in nuclei.

Sphingolipids

A significant corpus of work over the last decade has firmly established an important role for sphingolipids in a variety of important biological processes. Such processes include signaling events related to cell growth, differentiation, programmed cell death, and stress responses. These processes not only involve those sphingolipids that accumulate as a result of a variety of inherited lysosomal storage disorders, but, in addition, sphingolipids associated with long-chain base metabolism. This article reviews the chemical properties, pathways, regulated metabolism, and signaling function of sphingolipids. In addition, the potential roles of sphingolipids in renal-specific processes are considered. While a variety of cellular functions have been ascribed to sphingolipids, in many cases proof of the concept has yet to be well established. Thus, a number of critical questions can be posed in interpreting these studies. Several of these questions are considered.

Studies on a novel DNA polymerase inhibitor group, synthetic sulfoquinovosylacylglycerols: inhibitory action on cell proliferation

Some chemically synthesized sulfoquinovosylmonoacylglycerols (SQMG)-sulfoquinovosyldiacylglycerols (SQDG) have been reported to selectively and strongly inhibit the activities of mammalian DNA polymerases alpha and beta in vitro. In this study, using human cancer cell lines, we investigated the effects of SQMG-SQDG on the DNA polymerase in the cells. In the presence of n-decane, the IC(50) values on cell growth were approximately 1-5 microM for SQMG and about 0.3-1 microM for SQDG. The values were almost the same as the in vitro enzyme inhibitory levels. The cell lines were arrested in early S-phase by SQMG-SQDG at the concentrations of 0.1-4.7 microM in a manner dependent on incubation time, suggesting that SQMG-SQDG blocked the primary step of DNA replication by inhibiting DNA polymerase, possibly alpha-type. We also demonstrated the localization of SQMG in the cell using the fluorescent SQMG analog, SQMGalpha-NBDD, which was synthesized in our laboratory. SQMGalpha-NBDD was localized in the nucleus and on the nuclear surface, but the binding site seemed not to be the DNA/chromatin, suggesting that the SQMG-SQDG might interact with molecules located close to the DNA/chromatin and on the nuclear surface. These results suggested a correlation between the in vitro biochemical action of the SQMG-SQDGs and their intracellular mode of action.

Ethanol potentiates tumor necrosis factor-alpha cytotoxicity in hepatoma cells and primary rat hepatocytes by promoting induction of the mitochondrial permeability transition

In the present study, tumor necrosis factor-alpha (TNF-alpha) cytotoxicity is shown to be potentiated by ethanol exposure in vitro in the human hepatoma cell line, HepG2, and in rat primary hepatocytes. Exposure of HepG2 cells and primary hepatocytes for 48 hours to concentrations of ethanol ranging between 50 and 100 mmol/L significantly increased TNF-alpha cytotoxicity compared with cells treated with TNF-alpha alone. The cell killing was associated with, and dependent on, the development of the mitochondrial permeability transition (MPT). Two inhibitors of MPT pore opening, cyclosporin A and bongkrekic acid, prevented TNF-alpha cytotoxicity in the presence of ethanol. In addition to inhibiting cell death caused by TNF-alpha, blockade of MPT pore opening prevented mitochondrial depolarization, cytochrome c redistribution from the mitochondria to the cytosol, caspase 3 activation, and oligonucleosomal DNA fragmentation. Unlike the potentiation of TNF-alpha cytotoxicity by the translational inhibitor cycloheximide, ethanol promoted TNF-alpha-induced cell killing by a mechanism that was independent of caspase-8 activity. HepG2 cells overexpressing cytochrome-P4502E1 were even more sensitized by ethanol to induction of the MPT by TNF-alpha and the resultant cytotoxicity than wild-type HepG2 cells. In addition, primary hepatocytes isolated from chronically ethanol-fed rats showed enhanced susceptibility to TNF-alpha cytotoxicity compared with their isocalorically matched controls. Again as with the HepG2 cells, inhibiting MPT pore opening prevented the cytotoxicity of TNF-alpha in the primary hepatocytes isolated from ethanol-fed animals.

Lipid microsphere preparation of a lipophilic ceramide derivative suppresses colony formation in a murine experimental pulmonary metastasis model

Ceramide is a well-known regulator of apoptosis and cell growth. In this study, we synthesized lipophilic ceramide derivatives to incorporate into lipid microspheres (LM) and their activity was evaluated in vivo. Cera 03, a lipophilic ceramide derivative synthesized from membrane-permeable C2-ceramide, caused potent growth inhibition and DNA fragmentation of Meth A-T tumor cells in vitro. Its potency was similar to that of C2-ceramide. Both compounds increased the proportion of apoptotic cells. Cera 02, the diacetylated form of natural ceramide (Cer), also suppressed in vitro cell growth with a similar or higher potency to that of Cer, but both were far less potent than C2-ceramide and Cera 03. LM containing Cera 03 (Lipo-Cera 03) could not totally prevent metastatic incidence of Meth A-T cells, but reduced pulmonary metastatic nodules in number. Intravenous injection of Lipo-Cera 03 (1 mg/kg of Cera 03) produced about 35% inhibition, while Lipo-Cera 02 had no significant effect. In conclusion, Lipo-Cera 03 may have potential as an antimetastatic drug and may also be a useful tool for researching the role of ceramides in vivo.

CD95(Fas/APO-1) signals ceramide generation independent of the effector stage of apoptosis

Although numerous studies document caspase-independent ceramide generation preceding apoptosis upon environmental stress, the molecular ordering of ceramide generation during cytokine-induced apoptosis remains uncertain. Here, we show that CD95-induced ceramide elevation occurs during the initiation phase of apoptosis. We titrated down the amount of FADD transfected into HeLa and 293T cells until it was insufficient for apoptosis, although cycloheximide (CHX) still triggered the effector phase. Even in the absence of CHX, ceramide levels increased rapidly, peaking at 2.7 +/- 0.2-fold of control 8 h post-transfection. Dominant negative FADD failed to confer ceramide generation or CHX-mediated apoptosis. Ceramide generation induced by FADD was initiator caspase-dependent, being blocked by crmA. Limited pro-caspase 8 overexpression also increased ceramide levels 2.7 +/- 0.2-fold, yet failed, without CHX, to initiate apoptosis. Expression of membrane-targeted oligomerized CD-8 caspase 8 induced apoptosis without CHX, yet elevated ceramide only to a level equivalent to limited pro-caspase 8 transfection. Ceramide elevations were detected concurrently by diacylglycerol kinase and electrospray tandem mass spectrometry. These investigations provide evidence that ceramide generation is initiator caspase-dependent and occurs prior to commitment to the effector phase of apoptosis, definitively ordering ceramide as proximal in CD95 signaling.

Ceramide in the eukaryotic stress response

Several extracellular agents and stress stimuli, such as tumour necrosis factor alpha, chemotherapeutic agents and heat, cause ceramide accumulation. They do this by regulating enzymes involved in its metabolism. Ceramide modulates a number of biochemical and cellular responses to stress, including apoptosis, cell-cycle arrest and cell senescence.

N-acylated serinol is a novel ceramide mimic inducing apoptosis in neuroblastoma cells

A novel structural analog of ceramide was synthesized by N-acylation of serinol (2-amino-1,3-propanediol) and studied for its effects on glycolipid biosynthesis and cell differentiation of neuroblastoma cells. Incubation with N-palmitoylated serinol (C16-serinol) increased the concentration of endogenous ceramide by 50-80% and caused apoptosis in rapidly dividing low density cells but not in confluent cells. Cell death was not suppressed by simultaneous incubation with phorbol ester, known to antagonize ceramide-induced apoptosis by activation of protein kinase C (PKC). Purification of potential target proteins of C16-serinol was achieved by affinity chromatography of a protein preparation from rat brain on immobilized C16-serinol. A gel activity assay revealed that the eluate from C16-serinol-Sepharose contained three serine/threonine-specific protein kinases with molecular masses of 50, 70, and 95 kDa. The 70-kDa protein was immunostained on a Western blot using a PKCzeta-specific antibody. The purified PKCzeta could be activated directly by C16-serinol in an in vitro phosphorylation assay. Induction of apoptosis in neuroblastoma cells was suppressed by inhibition of PKCzeta with Gö 6983. Our overall results indicate that apoptosis in neuroblastoma cells induced by C16-serinol was at least partially mediated by activation of PKCzeta on condition of ongoing cell division. N-Acylated serinols may thus be useful for induction of apoptosis in mitotic cells and may be of therapeutic potential for treatment of cancer in the nervous system.

Tumor necrosis factor alpha signaling pathway and apoptosis in pancreatic beta cells

Cytokines induce apoptosis in pancreatic beta cells, but the exact mechanisms and sequence of events are not clear. Here, we investigate a role for tumor necrosis factor alpha (TNF-alpha) in the apoptosis of beta cells. Using the ribonuclease (RNase) protection assay and the reverse transcriptase-polymerase chain reaction (RT-PCR) method, we confirmed that TNF receptor 1 (TNFR1), TNFR1-associated death domain protein (TRADD), Fas receptor-associated intracellular protein with death domain (FADD), and FADD-like interleukin-1beta-converting enzyme (FLICE) were expressed in the pancreatic beta cell line, MIN6 cells. Fluorescent microscopic examination using Hoechst 33342 dye (Sigma, St Louis, MO) demonstrated that TNF-alpha induced time- and dose-dependent apoptotic nuclear changes in these beta cells. In situ end-labeling (ISEL) DNA analysis revealed that 10 nmol/L TNF-alpha generated new 3'-OH DNA strand breaks. Moreover, qualitative assessment of the induced DNA damage on agarose gels showed that 10 nmol/L TNF-alpha produced characteristic apoptotic patterns of DNA fragments formed by internucleosomal hydrolysis of static chromatin. In addition, C2-ceramides and natural ceramides dispersed in a solvent mixture of ethanol and dodecane induced characteristic features of apoptosis in MIN6 cells, mimicking TNF-induced DNA damage. We also determined endosomal ceramide production after TNF-alpha (10 nmol/L) treatment in MIN6 cells using the diacylglycerol kinase assay. These results suggest that TNF-alpha can cause apoptosis in pancreatic beta cells through TNFR1-linked apoptotic factors, TRADD, FADD, and FLICE, and TNF-induced ceramide production may be involved in the pathways.

Ceramide prevents motoneuronal cell death through inhibition of oxidative signal

We previously reported that cell death of rat spinal motoneurons, induced by trophic factor-deprivation, was attenuated by the application of exogenous cell-permeable ceramide (C6-Cer), or bacterial sphingomyelinase (SMase). Recently, motoneuronal cell death was demonstrated to be mediated by the generation of reactive oxygen species (ROS), including superoxide and peroxinitrite. In this study, to investigate the protective mechanism of ceramide (Cer), we examined the effects of Cer and sphingolipid metabolites against ROS generation and oxidative injury in enriched motoneuron cultures. Staining with C-DCDHF-DA, a fluorescent probe for detection of ROS, demonstrated that application of C6-Cer (2.5 mM) or bacterial SMase inhibited the increase of ROS generation. C6-dihydro-Cer, a biologically inactive analogue of C6-Cer, sphingosine, and sphingosine-1-phosphate did not affect ROS generation. This specificity corresponded to the results of cell survival assays. In addition, C6-Cer was shown to specifically inhibit ROS-induced reactions, such as tyrosine nitration and lipid peroxidation, in studies using antibodies against peroxinitrite and 4-hydroxinonenal, respectively. A potent neurotrophin for motoneurons, GDNF, had inhibitory effects against ROS generation and ROS-induced reactions. C6-Cer was also effective in the prevention of cytotoxicity induced by 1-buthionine-sulfoximine, an inhibitor of glutathione synthesis. These observations suggest that Cer plays a protective role in spinal motoneurons through inhibition of oxidative signals.

12-O-tetradecanoylphorbol-13-acetate-induced apoptosis is mediated by tumor necrosis factor alpha in human monocytic U937 cells

12-O-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester that is known as a tumor promoter, induces differentiation of myeloid cells and suppresses their proliferation. We studied the regulation of apoptosis by TPA in human monocytic cell line U937 cells that lack p53. Untreated U937 cells constitutively underwent apoptosis, and TPA enhanced apoptosis in these cells. Further studies showed that TPA increased production of tumor necrosis factor-alpha (TNFalpha) in U937 cells, and exogenously added TNFalpha induced apoptosis. Moreover, the induction of apoptosis by TPA was blocked by anti-TNFalpha antibody. Similar results were obtained in the myeloblastic cell line KY821 cells. We also found that the induction of apoptosis by TPA was increased in cells overexpressed with TNF receptor 1 but not in control cells. Furthermore, TPA failed to induce the production of TNFalpha and apoptosis in cells with either their protein kinase C or mitogen-activated protein kinase pathway blocked. Our results indicate that TPA induces apoptosis, at least in part, through a pathway that requires endogenous production of TNFalpha in U937 cells. Our data also suggest that the induction of apoptosis by TPA occurs through activation of protein kinase C and mitogen-activated protein kinase and TNFalpha is an autocrine-stimulating factor for the induction of apoptosis in these cells.

Epidermal growth factor inhibits ceramide-induced apoptosis and lowers ceramide levels in primary placental trophoblasts

The activation of sphingomyelinase and the subsequent generation of ceramide are emerging as important components of signaling pathways leading to apoptosis. The combination of tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) induces apoptosis of primary placental trophoblasts in vitro. This apoptosis is inhibited completely by cotreatment with epidermal growth factor (EGF). We therefore examined the role of sphingomyelinase and ceramide in trophoblast apoptosis and how this may be influenced by EGF. Exogenous C16-ceramide (20 microM) and acid sphingomyelinase induced trophoblast apoptosis, an effect abrogated completely by cotreatment with 10 ng/ml EGF. Neutral sphingomyelinase also increased ceramide levels but did not induce apoptosis. Treatment with EGF alone decreased cellular ceramide levels. This decrease could be blocked by cotreatment with the acid ceramidase inhibitor N-oleoylethanolamine (OE). OE alone increased ceramide levels and induced apoptosis that could not be blocked by cotreatment with EGF. In contrast, the alkaline ceramidase inhibitor D-MAPP, although it also increased ceramide levels, did not induce apoptosis nor did it affect TNF-alpha/IFN-alpha-induced cell death. These results implicate sphingolipids as important mediators in trophoblast apoptosis and suggest that the antiapoptotic properties of EGF can in part be explained by its control of ceramide concentrations in trophoblasts.

Long chain ceramides activate protein phosphatase-1 and protein phosphatase-2A. Activation is stereospecific and regulated by phosphatidic acid

The search for potential targets for ceramide action led to the identification of ceramide-activated protein phosphatases, which include protein phosphatase-2A (PP2A) and protein phosphatase-1 (PP1) with roles in regulating apoptosis and cell growth. Thus far, in vitro studies on ceramide-activated protein phosphatases have been restricted to the use of short chain ceramides, limiting the extent of mechanistic insight. In this study, we show that the long chain D-erythro-C18-ceramide activated PP2A (AB'C trimer), PP2Ac (catalytic subunit of PP2A), and PP1gammac and -alphac (catalytic subunits of PP1gamma and -1alpha isoforms, respectively) 2-6-fold in the presence of dodecane, a lipid-solubilizing agent, with 50% maximal activation achieved at approximately 10 microM D-erythro-C18-ceramide. The diastereoisomers of D-erythroC18-ceramide, D-threo-, and L-threo-C18-ceramide, as well as the enantiomeric L-erythro-C18-ceramide, did not activate PP1 or PP2A, but they inhibited PP1 and PP2A activity. The addition of phosphatidic acid decreased the basal activity of PP1c but also increased the stimulation by D-erythro-C18-ceramide from 1.8- to 2. 8-fold and decreased the EC50 of D-erythro-C18-ceramide to 4.45 microM. The addition of 150 mM KCl decreased the basal activity of PP1 and the dose of D-erythro-C18-ceramide necessary to activate PP1c (EC50 = 6.25 microM) and increased the ceramide responsiveness up to 10-17-fold. These studies disclose stereospecific activation of PP1 and PP2A by long chain natural ceramides under near physiologic ionic strengths in vitro. The implications of these studies for mechanisms of ceramide action are discussed.

Sphingosine inhibits nitric oxide synthase from cerebellar granule cells differentiated in vitro

The effects of different bioactive sphingoid molecules on NOS activity of differentiated cerebellar granule cells were investigated by measuring the conversion of [3H]arginine to [3H]citrulline. Cytosolic Ca2+-dependent NOS activity was strongly inhibited in a dose-dependent manner by sphingosine in concentrations of 1-40 microM. This inhibition seems to be peculiar to sphingosine in that ceramide, N-acetylsphingosine, sphingosine-1P, sphinganine and tetradecylamine have no effect on the cytosolic enzyme at the considered concentrations, suggesting that it is the bulk of the sphingosine hydrophilic portion that is critical for cytosolic NOS inhibition. This inhibition of cytosolic NOS is not reversed by increasing the arginine concentration, so a competitive mechanism can be excluded. Instead, increasing the concentrations of calmodulin led to loss of sphingosine inhibition, suggesting that sphingosine interferes with the calmodulin-dependent activation of the enzyme by a competitive mechanism. Sphingosine and related compounds had no effect on the particulate Ca2+-independent NOS activity. The data obtained suggest that sphingosine could be involved in the regulation of NO production in neurons.

Paclitaxel-induced apoptosis in Jurkat, a leukemic T cell line, is enhanced by ceramide

We hypothesized that the lipid second messenger, ceramide, and microtubule-directed chemotherapeutic agents might engage converging pathways in inducing apoptosis. Our studies demonstrated that simultaneous treatment of Jurkat cells with paclitaxel and ceramide enhanced paclitaxel-induced cell growth inhibition. Cell cycle analysis indicated a significant increase in the hypodiploid population over that observed with paclitaxel treatment alone. Morphologic evaluation and a TUNEL assay confirmed a dramatic increase in apoptosis in Jurkat cells treated with the combination of these two agents. This is the first demonstration that paclitaxel and ceramide interact in a supra-additive manner to decrease leukemic T-cell growth, suggesting a possible application of paclitaxel and ceramide in combination therapy.

Ceramide induces apoptosis to immature cerebellar granule cells in culture

A recent study revealed that ceramide acts as a second messenger in the sphingomyelin pathway and thus plays an important regulatory role in programmed cell death (apoptosis) to cell the lines induced by tumor-necrosis factor (TNF)-alpha and interleukin (IL)-1beta, although its effect remains controversial regarding primary neuronal culture. We investigated the effect of a cell-permeable ceramide analog (C2-ceramide) on cultures of cerebellar granule cells, which is thought to have active sphingomyelin pathway during development. The presence of C2-ceramide decreased the number of cerebellar granule cells (CGCs) in a concentration-dependent manner when added at DIV 1 (1 day in vitro). The ED50 was 60 microM. After DIV 2, CGCs became less sensitive to C2-ceramide and the ED50 was 200 microM at DIV 7. DNA staining with Hoechst 33258 showed the morphology of apoptotic nuclei in the degenerating neurons. Internucleosomal DNA degradation could also be observed by gel electrophoresis. Protein and RNA synthesis inhibitors prevented the death of neurons. C2-dihydroceramide, which lacks the 4-5 trans double bond and failed to induce neuronal death. These results thus demonstrated that C2-ceramide induces apoptosis to the CGCs at the early stage in vitro, however the CGCs were found to be less sensitive to C2-ceramide at the later stage in vitro.

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

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

Acyl chain length-specific ceramide-induced changes in intracellular Ca2+ concentration and progesterone production are not regulated by tumor necrosis factor alpha in hen granulosa cells

Although tumor necrosis factor alpha (TNF-alpha) has long been known to be a potent inhibitor of gonadotropin-induced cytodifferentiation in the ovaries of a variety of mammalian species, its early signal transduction events are poorly understood. We previously demonstrated that TNF-alpha induces a small, delayed follicular stage-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) in hen granulosa cells and promotes carbachol (Cch)-induced mobilization of Ca2+ from intracellular stores in cells otherwise unresponsive to the cytokine. The focus of the current study was to examine the role of ceramide in TNF-alpha-induced Ca2+ regulation. Treatment with exogenous sphingomyelinase (SMase; 50 mU/ml) failed to influence basal [Ca2+]i but increased the magnitude of Cch-induced Ca2+ transients. While C8-ceramide (0.03-30 microM), but not C2-ceramide (0.03-30 microM), mimicked this effect of SMase, challenge with sphingosine (3 microM) resulted in a slow and delayed increase in basal [Ca2+]i. In order to determine whether SMase is activated by TNF-alpha action, changes in sphingomyelin and ceramide concentrations in F1 and F5,6 granulosa cells were determined. SMase activation was not observed after 1-, 5-, 15-, and 60-min incubations with TNF-alpha (1-50 ng/ml) in either F1 or F5,6 cells. Exogenous SMase and C2-ceramide both inhibited LH-induced progesterone production in F1 and F5,6 cells; however, incubation with C8-ceramide resulted in increases in both basal and LH-induced progesterone. In contrast, incubation with TNF-alpha had no effect on either basal or LH-induced steroidogenesis. In conclusion, our findings indicate that although ceramide regulates [Ca2+]i and progesterone secretion, the sphingolipid does not appear to play a role in the action of TNF-alpha in avian granulosa cells. Furthermore, ceramide-mediated responses are highly dependent on acyl chain length, potentially reflecting differences in the abilities of these ceramides to access, bind to, and/or activate ceramide-dependent signal transduction mechanisms. Nonetheless, since TNF-alpha did not increase the production of ceramide, the physiological regulator(s) of these responses remain unknown.

Antigenicity of the carbohydrate moiety of ganglioside GM3 having 3-O-acetyl ceramide

To elucidate the effect of a modification of ceramide on antigenicity of the carbohydrate of ganglioside, the reactivity of O-acetyl GM3 having 3-O-acetyl ceramide, which has been characterized as a glioma-related ganglioside, with monoclonal antibody M2590 was examined in comparison to that of non-acetylated GM3, by means of quantitative enzyme-linked immunosorbent assay, TLC-immunostaining and liposome immune lysis assay. In all these assay systems, O-acetyl GM3 showed less activity than GM3 as follows: GM3 was detected till 0.1 nmol in TLC-immunostaining, whereas O-acetyl GM3 could not be detected even at 0.25 nmol; the GM3 reaction was approximately twofold that of O-acetyl GM3 at each diluted point in the enzyme-linked immunosorbent assay; and 20% of the liposomes containing GM3 were lysed at 6 mol%, while liposomes containing O-acetyl GM3 did not lyse at that concentration. The lesser antigenicity of the sugar moiety of O-acetyl GM3 could be ascribed to the presence of an acetyl group in the ceramide at the 3-position of sphingosine.

Application of exogenous ceramide to cultured rat spinal motoneurons promotes survival or death by regulation of apoptosis depending on its concentrations

The membrane lipid ceramide (Cer) has been shown to be involved in the survival and dendritic growth of cerebellar Purkinje cells and hippocampal neurons. We examined the effects of Cer on isolated rat spinal motoneurons. Basal neuronal cell death due to apoptosis occurs under these culture conditions. This cell death was prevented by treatment with 2.5 microM of D-erythro-N-hexsanoylsphingosine (C6-Cer), a cell-permeable analogue, and the surviving cell number was increased approximately 1.6-fold compared with the control cell number on 5 days in vitro (DIV). Application of the same amount of C6-Cer improved axonal elongation. Conversely, addition of 10 microM of C6-Cer led all motoneurons to apoptotic cell death by 2DIV. A stereo isomer, threo-C6-Cer, which is not metabolized to C6-glucosylceramide, also promoted survival, death, and axonal growth in the same manners as C6-Cer. However, C6-dihydro-Cer, a biologically inactive analogue, had no effects on survival or death, indicating that the presence of a double bond in the sphingosine base is essential for its activity. In addition, treatment with bacterial sphingomyelinase, which generates endogenous Cer, increases motoneuron survival and axonal growth. These observations suggest that Cer, but not its metabolites, regulates survival and development of spinal motoneurons, depending on its intracellular concentration.

Diversity and complexity of ceramide signalling in apoptosis

Sphingolipid ceramide has emerged as a lipid messenger of cell functions including differentiation and apoptosis. Diverse kinds of stresses (ultraviolet, irradiation, heat shock and hypoxia) and biological factors (TNF-alpha, IFN-gamma and Fas antibody) require ceramide generation to execute apoptosis. The review summarises the diversity and complexity of up- and downstream of ceramide signalling in apoptosis and clinical implications of ceramide-induced apoptosis.

Evidence that protein kinase Cepsilon mediates phorbol ester inhibition of calphostin C- and tumor necrosis factor-alpha-induced apoptosis in U937 histiocytic lymphoma cells

Protein kinase C (PKC) activators, such as the tumor-promoting phorbol esters, have been reported to protect several cell lines from apoptosis induced by a variety of agents. Recent evidence suggests that PKCepsilon is involved in protection of cardiac myocytes from hypoxia-induced cell death (Gray, M. O., Karliner, J. S., and Mochly-Rosen, D. (1997) J. Biol. Chem. 272, 30945-30951). We investigated the protective effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on U937 histiocytic lymphoma cells induced to undergo apoptosis by tumor necrosis factor-alpha (TNF-alpha) or by the specific PKC inhibitor calphostin C. U937 cells were transiently permeabilized with a peptide (epsilonV1-2) derived from the V1 region of PKCepsilon that has been reported to specifically block translocation of PKCepsilon. The epsilonV1-2 peptide blocked the inhibitory effect of TPA on both TNF-alpha- and calphostin C-induced apoptosis. A scrambled version of epsilonV1-2 and a peptide reported to inhibit PKCbeta translocation (betaC2-4) had no effect on the ability of TPA to inhibit apoptosis. These results suggest that PKCepsilon is required for the protective effect of TPA in TNF-alpha- and calphostin C-induced apoptosis. Furthermore, calphostin C reduced membrane-associated PKCepsilon activity and immunoreactivity, suggesting that PKCepsilon may play an important role in leukemic cell survival.