Dissociation of endogenous cellular ceramide from NF-kappa B activation

The Herbal Bitter Drug Gentiana lutea Modulates Lipid Synthesis in Human Keratinocytes In Vitro and In Vivo

Gentiana lutea is a herbal bitter drug that is used to enhance gastrointestinal motility and secretion. Recently we have shown that amarogentin, a characteristic bitter compound of Gentiana lutea extract (GE), binds to the bitter taste receptors TAS2R1 and TAS2R38 in human keratinocytes, and stimulates the synthesis of epidermal barrier proteins. Here, we wondered if GE also modulates lipid synthesis in human keratinocytes. To address this issue, human primary keratinocytes were incubated for 6 days with GE. Nile Red labeling revealed that GE significantly increased lipid synthesis in keratinocytes. Similarly, gas chromatography with flame ionization detector indicated that GE increases the amount of triglycerides in keratinocytes. GE induced the expression of epidermal ceramide synthase 3, but not sphingomyelinase. Lipid synthesis, as well as ceramide synthase 3 expression, could be specifically blocked by inhibitors of the p38 MAPK and PPARγ signaling pathway. To assess if GE also modulates lipid synthesis in vivo, we performed a proof of concept half side comparison on the volar forearms of 33 volunteers. In comparison to placebo, GE significantly increased the lipid content of the treated skin areas, as measured with a sebumeter. Thus, GE enhances lipid synthesis in human keratinocytes that is essential for building an intact epidermal barrier. Therefore, GE might be used to improve skin disorders with an impaired epidermal barrier, e.g., very dry skin and atopic eczema.

Glycosphingolipids and cell death: one aim, many ways

Glycosphingolipids (GSLs) are a family of bioactive lipids that in addition to their role in the regulation of structural properties of membrane bilayers have emerged as crucial players in many biological processes and signal transduction pathways. Rather than being uniformly distributed within membrane bilayers, GSLs are localized in selective domains called lipid rafts where many signaling platforms operate. One of the most important functions of GSLs, particularly ceramide, is their ability to regulate cell death pathways and hence cell fate. This complex role is accomplished by the ability of GSLs to act in distinct subcellular strategic centers, such as mitochondria, endoplasmic reticulum (ER) or lysosomes to mediate apoptosis, ER stress, autophagy, lysosomal membrane permeabilization and necroptosis. Hence better understanding the role of GSLs in cell death may be of relevance for a number of pathological processes and diseases, including neurodegeneration, metabolic liver diseases and cancer.

Regulation of exosome release by glycosphingolipids and flotillins

Exosomes are released by cells after fusion of multivesicular bodies with the plasma membrane. The molecular mechanism of this process is still unclear. We investigated the role of sphingolipids and flotillins, which constitute a raft-associated family of proteins, in the release of exosomes. Interestingly, our results show that dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, an inhibitor of glucosylceramide synthase, seemed to affect the composition of exosomes released from PC-3 cells. However, the inhibition of ceramide formation from the de novo pathway by fumonisin B1 did not affect exosome secretion. Moreover, in contrast to findings obtained with other cell lines published so far, inhibition of neutral sphingomyelinase 2, an enzyme that catalyzes the formation of ceramide from sphingomyelin, did not inhibit the secretion of exosomes in PC-3 cells. Finally, small interfering RNA-mediated downregulation of flotillin-1 and flotillin-2 did not significantly change the levels of released exosomes as such, but seemed to affect the composition of exosomes. In conclusion, our results reveal the involvement of glycosphingolipids and flotillins in the release of exosomes from PC-3 cells, and indicate that the role of ceramide in exosome formation may be cell-dependent.

Ceramide inhibits IL-2 production by preventing protein kinase C-dependent NF-kappaB activation: possible role in protein kinase Ctheta regulation

The role of the sphingolipid ceramide in modulating the immune response has been controversial, in part because of conflicting data regarding its ability to regulate the transcription factor NF-kappaB. To help clarify this role, we investigated the effects of ceramide on IL-2, a central NF-kappaB target. We found that ceramide inhibited protein kinase C (PKC)-mediated activation of NF-kappaB. Ceramide was found to significantly reduce the kinase activity of PKCtheta as well as PKCalpha, the critical PKC isozymes involved in TCR-induced NF-kappaB activation. This was followed by strong inhibition of IL-2 production in both Jurkat T leukemia and primary T cells. Exogenous sphingomyelinase, which generates ceramide at the cell membrane, also inhibited IL-2 production. As expected, the repression of NF-kappaB activation by ceramide led to the reduction of transcription of the IL-2 gene in a dose-dependent manner. Inhibition of IL-2 production by ceramide was partially overcome when NF-kappaB nuclear translocation was reconstituted with activation of a PKC-independent pathway by TNF-alpha or when PKCtheta was overexpressed. Importantly, neither the conversion of ceramide to complex glycosphingolipids, which are known to have immunosuppressive effects, nor its hydrolysis to sphingosine, a known inhibitor of PKC, was necessary for its inhibitory activity. These results indicate that ceramide plays a negative regulatory role in the activation of NF-kappaB and its targets as a result of inhibition of PKC.

Significant role of ceramide pathway in experimental gastric ulcer formation in rats

Ceramides have emerged as key participants in the signaling pathway of cytokines and apoptosis. We previously revealed that phorbol 12-myristate 13-acetate (PMA) induced experimental ulcers in rat gastric mucosa. In this study, we investigated the role of ceramide in ulcer formation and its relation to the activation of transcription factors and apoptosis. PMA was subserosally injected to rat glandular stomach. Fumonisin B1 (FB1), an inhibitor of ceramide synthase, was administered together with the PMA. The time course of ceramide content was quantified using thin layer chromatography and the number of apoptotic cells was determined by immunohistochemistry. The activation of transcription factor nuclear factor-kappaB (NF-kappaB) or activator protein-1 (AP-1) was evaluated using an electrophoretic mobility shift assay. The administration of FB1 attenuated PMA-induced gastric ulcer formation in a dose-dependent manner. Before the ulcers became obvious, the ceramide content (C18 and C24 ceramide) increased significantly in the gastric wall. The activation of NF-kappaB and AP-1 and an increase in the number of apoptotic cells were also observed. Both of these were significantly inhibited by the coadministration of FB1. However, NF-kappaB inhibitors attenuated gastric ulcer formation without affecting the ceramide content or the number of apoptotic cells. Ceramide formation in the stomach significantly contributes to PMA-induced tissue damage, possibly via the activation of transcription factors and an increase in apoptosis in the gastric mucosa. However, after the increase in ceramide levels, the NF-kappaB and apoptosis pathways may be separately involved in ulcer formation.

Leustroducsin B activates nuclear factor-kappaB via the acidic sphingomyelinase pathway in human bone marrow-derived stromal cell line KM-102

The novel colony-stimulating factor (CSF) inducer leustroducsin B (LSN-B), which was isolated from Streptomyces platensis, has been shown to have potent cytokine-inducing activities in clonal human bone marrow-derived stromal cell line KM-102 and in primary human bone marrow-derived stromal cells. In this study, we investigated the signal transduction pathway of LSN-B using luciferase expression plasmids linked to the 5'-flanking region of interleukin-8 (IL-8) and that of the IL-11 gene. In KM-102 cells, LSN-B induced luciferase activity both in the wild-type and in the activated protein 1 (AP-1) site point-mutated IL-8 promoter. The mutation in the nuclear factor-kappaB (NF-kappaB) site abrogated LSN-B-stimulated induction of the reporter gene. LSN-B-inducing activity was inhibited by (1) N-acetyl-L-cysteine, a well-characterized antioxidant, (2) cationic amphiphilic drugs, inhibitors of acidic sphingomyelinase (A-SMase), and (3) D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC). These observations suggest that LSN-B potentiates the A-SMase-mediated signaling pathway to stimulate NF-kappaB. In contrast, LSN-B did not induce IL-11 promoter-driven luciferase activity. The observed increase in IL-11 mRNA stability by LSN-B indicates that the inducible production of IL-11 by LSN-B is regulated at the posttranscriptional level. In addition, inhibition of LSN-B-mediated induction of IL-11 production by cationic amphiphilic drugs and D609 in KM-102 cells demonstrates that increased IL-11 mRNA stability by LSN-B might be mediated via NF-kappaB activation. From these results, we suggest that LSN-B induces cytokine production through at least two separate mechanisms, at the transcriptional level and at the posttranscriptional level via NF-kappaB activation.

Sphingolipid mediators in cardiovascular cell biology and pathology

Sphingolipids have emerged as a new class of lipid mediators. In response to various extracellular stimuli, sphingolipid turnover can be stimulated in vascular cells and cardiac myocytes. Subsequent generation of sphingolipid molecules such as ceramide, sphingosine, and sphingosine-1-phosphate, is followed by regulation of ion fluxes and activation of various signaling pathways leading to smooth muscle cell proliferation, endothelial cell differentiation or apoptotic cell death, cell contraction, retraction, or migration. The importance of sphingolipids in cardiovascular signaling is illustrated by recent observations implicating them in physiological processes such as vasculogenesis as well as in frequent pathological conditions, including atherosclerosis and its complications.

Signaling pathways activated by daunorubicin

The anthracycline daunorubicin is widely used in the treatment of acute nonlymphocytic leukemia. The drug has, of course, been the object of intense basic research, as well as preclinical and clinical study. As reviewed in this article, evidence stemming from this research clearly demonstrates that cell response to daunorubicin is highly regulated by multiple signaling events, including a sphingomyelinase-initiated sphingomyelin-ceramide pathway, mitogen-activated kinase and stress-activated protein/c-Jun N-terminal kinase activation, transcription factors such as nuclear factor kappa B, as well as the Fas/Fas-ligand system. These pathways are themselves influenced by a number of lipid products (diacylglycerol, sphingosine-1 phosphate, and glucosyl ceramide), reactive oxygen species, oncogenes (such as the tumor suppressor gene p53), protein kinases (protein kinase C and phosphoinositide-3 kinase), and external stimuli (hematopoietic growth factors and the extracellular matrix). In light of the complexity and diversity of these observations, a comprehensive review has been attempted toward the understanding of their individual implication (and regulation) in daunorubicin-induced signaling. (Blood. 2001;98:913-924)

Ceramide inhibits lipopolysaccharide-mediated nitric oxide synthase and cyclooxygenase-2 induction in macrophages: effects on protein kinases and transcription factors

The goal of this study was to elucidate whether triggering the sphingomyelin pathway modulates LPS-initiated responses. For this purpose we investigated the effects of N-acetylsphingosine (C(2)-ceramide) on LPS-induced production of NO and PGE(2) in murine RAW 264.7 macrophages and explored the signaling pathways involved. We found that within a range of 10-50 microM, C(2)-ceramide inhibited LPS-elicited NO synthase and cyclooxygenase-2 induction accompanied by a reduction in NO and PGE(2) formation. By contrast, a structural analog of C(2)-ceramide that does not elicit functional activity, C(2)-dihydroceramide, did not affect the LPS response. The nuclear translocation and DNA binding study revealed that ceramide can inhibit LPS-induced NF-kappaB and AP-1 activation. The immunocomplex kinase assay indicated that IkappaB kinase activity stimulated by LPS was inhibited by ceramide, which concomitantly reduced the IkappaBalpha degradation caused by LPS within 1-6 h. In concert with the decreased cytosolic p65 protein level, LPS treatment resulted in rapid nuclear accumulation of NF-kappaB subunit p65 and its association with the cAMP-responsive element binding protein. Ceramide coaddition inhibited all the LPS responses. In addition, LPS-induced PKC and p38 mitogen-activated protein kinase activation were overcome by ceramide. In conclusion, we suggest that ceramide inhibition of LPS-mediated induction of inducible NO synthase and cyclooxygenase-2 is due to reduction of the activation of NF-kappaB and AP-1, which might result from ceramide's inhibition of LPS-stimulated IkappaB kinase, p38 mitogen-activated protein kinase, and protein kinase C.

Ceramide induction of COX-2 and PGE(2) in pulmonary A549 cells does not involve activation of NF-kappaB

Ceramide is generated by the hydrolysis of membrane sphingomyelin by sphingomyelinase (SMase) and is implicated in multiple signaling pathways, including activation of NF-kappaB. As NF-kappaB is pivotal in the expression of numerous genes associated with airway inflammation and asthma, the effects of ceramide and SMase were examined in human pulmonary A549 cells. Ceramide and SMase both induced cyclooxygenase (COX)-2 protein expression and stimulated PGE(2) release. However, neither ceramide nor SMase induced NF-kappaB DNA-binding, loss of IkappaBalpha, or NF-kappaB-dependent transcription. Both ceramide and SMase were efficient inducers of the extracellular regulated kinase (ERK), but not Jun N-terminal kinase (JNK) or p38 mitogen-activated protein kinase. Since ERK is implicated in arachidonic acid availability, these data partly explain the ability of ceramide to induce PGE(2) release. However, as ERK is not required for IL-1beta-dependent induction of COX-2, the mechanism of ceramide and SMase induction of COX-2 remains unclear.

Ceramide initiates NFkappaB-mediated caspase activation in neuronal apoptosis

The objective of the present study was to evaluate the role of ceramide in mediating apoptosis of dorsal root ganglion neurons induced by either nerve growth factor withdrawal or treatment with the chemotherapeutic agents suramin and cisplatin. Measurement of ceramide accumulation by mass spectrometry and the diacylglycerol kinase assay revealed elevation of intracellular ceramide only in suramin treated cultures. Ceramide-mediated neuronal cell death was inhibited by the caspase inhibitor zVAD.fmk. In these experimental models, ceramide accumulation mediated activation and nuclear translocation of the transcription factor NFkappaB and cyclin D1 protein expression. Specific inhibition of NFkappaB using a molecular decoy strategy resulted in increased cell viability accompanied by diminished caspase activity and cyclin D1 expression. Inhibition of NFkappaB did not alter intracellular ceramide levels. Our study suggests that ceramide generation occurs upstream of NFkappaB activation, cell cycle reentry, and caspase activation in the neuronal death pathway.

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.

Differential effects of sphingomyelin hydrolysis and resynthesis on the activation of NF-kappa B in normal and SV40-transformed human fibroblasts

The precise role of ceramide in NF-kappaB signaling remains unclear. The recent observation of differential sphingomyelin synthase (SMS) activity in normal (low SMS) versus SV40-transformed (high SMS) WI38 human lung fibroblasts provides an opportunity to assess the involvement of ceramide and SMS in NF-kappaB activation. Treatment of normal WI38 fibroblasts with bacterial sphingomyelinase resulted in a 4-fold elevation of ceramide and blocked NF-kappaB activation by serum stimulation. Such inhibition was not observed in SV40-transformed fibroblasts. Under regular growth conditions, after sphingomyelinase was washed out, normal WI38 did not show SM re-synthesis nor NF-kappaB activation. In SV40-WI38, on the other hand, sphingomyelinase washout induced resynthesis of SM due to the action of SMS on ceramide generated at the plasma membrane. NF-kappaB activation correlated with SM resynthesis. This activation was abrogated by D609, which inhibited SM resynthesis but not the initial formation of ceramide. The differential activity of SMS may explain the effects of ceramide in NF-kappaB signaling: in the absence of significant SMS activity, ceramide inhibits NF-kappaB, whereas with high SMS, the conversion of the ceramide signal to a diacylglycerol signal by the action of SMS stimulates NF-kappaB. These results also suggest a role for SMS in regulating NF-kappaB.

Protein tyrosine kinase p56lck is required for ceramide-induced but not tumor necrosis factor-induced activation of NF-kappa B, AP-1, JNK, and apoptosis

Ceramide has been implicated as an intermediate in the signal transduction of several cytokines including tumor necrosis factor (TNF). Both ceramide and TNF activate a wide variety of cellular responses, including NF-kappaB, AP-1, JNK, and apoptosis. Whether ceramide transduces these signals through the same mechanism as TNF is not known. In the present study we investigated the role of the T cell-specific tyrosine kinase p56(lck) in ceramide- and TNF-mediated cellular responses by comparing the responses of Jurkat T cells with JCaM1 cells, isogeneic Lck-deficient T cells. Treatment with ceramide activated NF-kappaB, degraded IkappaBalpha, and induced NF-kappaB-dependent reporter gene expression in a time-dependent manner in Jurkat cells but not in JCaM1 cells, suggesting the critical role of p56(lck) kinase. These effects were specific to ceramide, as activation of NF-kappaB by phorbol 12-myristate 13-acetate, lipopolysaccharide, H(2)O(2), and TNF was minimally affected. p56(lck) was also found to be required for ceramide-induced but not TNF-induced AP-1 activation. Similarly, ceramide activated the protein kinases JNK and mitogen-activated protein kinase kinase in Jurkat cells but not in JCaM1 cells. Ceramide also induced cytotoxicity and activated caspases and reactive oxygen intermediates in Jurkat cells but not in JCaM1 cells. Ceramide activated p56(lck) activity in Jurkat cells. Moreover, the reconstitution of JCaM1 cells with p56(lck) tyrosine kinase reversed the ceramide-induced NF-kappaB activation and cytotoxicity. Overall our results demonstrate that p56(lck) plays a critical role in the activation of NF-kappaB, AP-1, JNK, and apoptosis by ceramide but has minimal or no role in activation of these responses by TNF.

Divergence in regulation of nitric-oxide synthase and its cofactor tetrahydrobiopterin by tumor necrosis factor-alpha. Ceramide potentiates nitric oxide synthesis without affecting GTP cyclohydrolase I activity

Synthesis of 6(R)-5,6,7,8-tetrahydrobiopterin (BH(4)), a required cofactor for inducible nitric-oxide synthase (iNOS) activity, is usually coordinately regulated with iNOS expression. In C6 glioma cells, tumor necrosis factor-alpha (TNF-alpha) concomitantly potentiated the stimulation of nitric oxide (NO) and BH(4) production induced by IFN-gamma and interleukin-1beta. Expression of both iNOS and GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in the BH(4) biosynthetic pathway, was also markedly increased, as were their activities and protein levels. Ceramide, a sphingolipid metabolite, may mediate some of the actions of TNF-alpha. Indeed, we found that bacterial sphingomyelinase, which hydrolyzes sphingomyelin and increases endogenous ceramide, or the cell permeable ceramide analogue, C(2)-ceramide, but not C(2)-dihydroceramide (N-acetylsphinganine), significantly mimicked the effects of TNF-alpha on NO production and iNOS expression and activity in C6 cells. Surprisingly, although TNF-alpha increased BH(4) synthesis and GTPCH activity, neither BH(4) nor GTPCH expression was affected by C(2)-ceramide or sphingomyelinase in IFN-gamma- and interleukin-1beta-stimulated cells. It is likely that increased BH(4) levels results from increased GTPCH protein and activity in vivo rather than from reduced turnover of BH(4), because the GTPCH inhibitor, 2,4-diamino-6-hydroxypyrimidine, blocked cytokine-stimulated BH(4) accumulation. Moreover, expression of the GTPCH feedback regulatory protein, which if decreased might increase GTPCH activity, was not affected by TNF-alpha or ceramide. Treatment with the antioxidant pyrrolidine dithiocarbamate, which is known to inhibit NF-kappaB and sphingomyelinase in C6 cells, or with the peptide SN-50, which blocks translocation of NF-kappaB to the nucleus, inhibited TNF-alpha-dependent iNOS mRNA expression without affecting GTPCH mRNA levels. This is the first demonstration that cytokine-stimulated iNOS and GTPCH expression, and therefore NO and BH(4) biosynthesis, may be regulated by discrete pathways. As BH(4) is also a cofactor for the aromatic amino acid hydroxylases, discovery of distinct mechanisms for regulation of BH(4) and NO has important implications for its specific functions.

Nerve growth factor induces sphingomyelin accumulation in pheochromocytoma cells

The pheochromocytoma cells are a well-known model for studying the nerve growth factor (NGF)-induced molecular changes during the differentiation process. The involvement of sphingomyelin (SM) was studied using the fluorescent analogue of ceramide, i.e. N-lissamine rhodaminyl-(12-aminododecanoyl) D-erythro-sphingosine (C12-LRh-Cer). This fluorescent analogue is metabolically active and can be used to follow the biosynthesis of SM in intact cells. NGF induces a 4-fold increase of fluorescent SM content in PC12 cells, when loaded with C12-LRh-Cer. Treatment of PC12 cells with actinomycin D or cycloheximide completely abolishes the NGF-induced elevation of SM. Inhibition of p140(trkA) receptor by AG-879 prevents extracellular signal-regulated kinase 1/2 phosphorylation and suppresses the increase of SM. Inhibition of protein kinase C (PKC), protein kinase A (PKA) and phosphatidylinositol 3-kinase does not have any effect on NGF-induced C12-LRh-SM accumulation. On the other hand, activation of PKA or PKC with simultaneous treatment with NGF has a synergistic effect on increase of SM content. The NGF-induced SM increase in PC12 cells is an effect promoted by other differentiating agents like dibutyryl cyclic AMP or fibroblast growth factor-2 but not by a mitogenic agent like epidermal growth factor.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

Ceramide synergizes with phorbol ester or okadaic acid to induce IkappaB degradation

Ceramide is a lipid second messenger which is generated in response to stimulation of a number of surface receptors, treatment with chemotherapeutic agents, or ionising radiation. Depending on the target cell, ceramide induces diverse biological responses including apoptosis, cell-cycle arrest, differentiation, and also proliferation. We studied the effect of ceramide on the degradation of IkappaB, the cytoplasmic inhibitor of the transcription factor NF-kappaB. We show that ceramide treatment results in reduced levels of phosphorylated IkappaBalpha and degradation of both IkappaBalpha and IkappaBbeta. Ceramide synergised with okadaic acid (OA), a compound which interferes with the protein phosphatase 2A-controlled component of the NF-kappaB activation pathway, enhancing OA-induced IkappaB degradation. Ceramide also synergised with phorbol 12-myristate 13-acetate, which mimics protein kinase C activation. Finally, we show that the synergistic effect of ceramide with OA or phorbol ester can be observed in primary lymph node T-cells as well as in transformed T-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.

Limited role of ceramide in lipopolysaccharide-mediated mitogen-activated protein kinase activation, transcription factor induction, and cytokine release

The involvement of ceramide in lipopolysaccharide-mediated activation of mouse macrophages was studied. Lipopolysaccharide, cell-permeable ceramide analogs, and bacterial sphingomyelinase led to phosphorylation of the extracellular signal-regulated kinases, c-Jun NH2-terminal kinases, and p38 kinase and induced AP-1 DNA binding in C3H/OuJ (Lpsn) but not in C3H/HeJ (Lpsd) macrophages. Lipopolysaccharide and ceramide mimetics showed distinct kinetics of mitogen-activated protein kinase phosphorylation and AP-1 induction and activated AP-1 complexes with different subunit compositions. Lipopolysaccharide-activated AP-1 consisted of c-Fos, Jun-B, Jun-D, and c-Jun, while C2-ceramide induced Jun-D and c-Jun only. Lipopolysaccharide and, less potently, C2-ceramide or sphingomyelinase, stimulated AP-1-dependent reporter gene transcription in RAW 264.7 cells. Unlike lipopolysaccharide, C2-ceramide failed to activate NF-kappaB and did not induce production of tumor necrosis factor or interleukin-6. The lipopolysaccharide antagonist, Rhodobacter sphae-roides diphosphoryl lipid A, inhibited lipopolysaccharide activation of NF-kappaB and AP-1 but did not block C2-ceramide-induced AP-1. Pretreatment of C3H/OuJ macrophages with C2-ceramide greatly diminished AP-1 induction following subsequent C2-ceramide stimulation. However, lipopolysaccharide-induced transcription factor activation and cytokine release were not influenced. In contrast, lipopolysaccharide pretreatment inhibited both lipopolysaccharide- and C2-ceramide-mediated responses. Thus, ceramide partially mimics lipopolysaccharide in activating the mitogen-activated protein kinases and AP-1 but not in mediating NF-kappaB induction or cytokine production, suggesting a limited role in lipopolysaccharide signaling.

Involvement of lipid mediators on cytokine signaling and induction of secretory phospholipase A2 in immortalized astrocytes (DITNC)

Our previous studies demonstrated the ability of proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin 1beta (IL-1beta), to stimulate NFkappaB/DNA binding and synthesis of secretory phospholipase A2 (sPLA2) in immortalized astrocytes (DITNC). In this study, we examined possible involvement of lipid mediators in the cytokine action. Using [14C]serine to label sphingomyelin and ceramide in these cells, subsequent exposure of cells to cytokines did not result in alteration of sphingomyelin/ceramide ratio. Furthermore, neither exogenous sphingomyelinase nor cell-permeable ceramides could stimulate NFkappaB/DNA binding. On the other hand, C-2 ceramide (0.3 microM) as well as other lipid mediators, such as lysophosphatidylcholine and arachidonic acid, were able to elicit a small increase in sPLA2 and potentiate the induction of sPLA2 by TNF-alpha. When DITNC cells were prelabeled with [32P]Pi, an increase in labeled phosphatidic acid (PA) was observed on treatment of cells with IL-1beta (200 U/mL). However, despite the ability of phorbol myristate acetate (PMA) to stimulate phospholipase D (PLD) and synthesis of phosphatidylethanol (PEt) in these cells, PLD activity was not affected by IL-1beta. With the [32P]labeled cells, however, PA-phosphohydrolase inhibitors, such as chlorpromazine and propranolol, could elicit large increases in labeled PA, indicating active PA metabolism in these cells. Cytokines also caused an increase in levels of diacylglycerol (DG) in these cells, although the source of this lipid pool is presently not understood. Taken together, these results provide evidence for the participation of PA and DG in cytokine signaling activity. Furthermore, although cytokines did not cause the release of ceramide, lipid mediators, such as lysophospholipids, and AA could modulate cytokine-mediated induction of sPLA2 in astrocytes.

Role of ceramide in lipopolysaccharide (LPS)-induced signaling. LPS increases ceramide rather than acting as a structural homolog

Ceramide and ceramide-activated enzymes have been implicated in responses to bacterial lipopolysaccharide (LPS) and the proinflammatory cytokines tumor necrosis factor-alpha (TNF) and interleukin-1beta (IL-1). Although TNF and IL-1 cause elevation of cellular ceramide, which is thought to act as a second messenger, LPS has been proposed to signal by virtue of structural similarity to ceramide. We have investigated the relationship between ceramide and LPS by comparing the effects of a cell-permeable ceramide analog (C2-ceramide) and LPS on murine macrophage cell lines and by measuring ceramide levels in macrophages exposed to LPS. We found that while both C2-ceramide and LPS activated c-Jun N-terminal kinase (JNK), only LPS also activated extracellular signal-regulated kinases (ERKs). C2-ceramide was also unable to activate NF-kappaB, a transcription factor important for LPS-induced gene expression. Upon measurement of cellular ceramide in macrophage lines, we observed a small but rapid rise in ceramide, similar to that seen upon IL-1 or TNF treatment, suggesting LPS induces an increase in ceramide rather than interacting directly with ceramide-responsive enzymes. We found that C2-ceramide activated JNK and induced growth arrest in macrophages cell lines from both normal mice (Lpsn) and mice genetically unresponsive to LPS (Lpsd), whereas only Lpsn macrophages made these responses to LPS. Surprisingly, LPS treatment of Lpsd macrophages induced a rise in ceramide similar to that observed in LPS-responsive cells. These results indicate that the wild type Lps allele is not required for LPS-induced ceramide generation and suggest that ceramide elevation alone is insufficent stimulus for most responses to LPS.

Lipopolysaccharide and Ceramide Use Divergent Signaling Pathways to Induce Cell Death in Murine Macrophages

Ceramide is a well-known apoptotic agent that has been implicated in LPS signaling. Therefore, we examined whether LPS-induced macrophage cytotoxicity is mediated by mimicking ceramide. Both LPS and the cell-permeable ceramide analogue, C2 ceramide, induced significant cell death in IFN-γ-activated, thioglycollate-elicited peritoneal macrophages after 48 and 24 h, respectively. Ceramide-induced cell death was neither accompanied by DNA fragmentation nor phosphatidyl serine externalization, characteristics of apoptosis. In contrast, LPS induced a significant fraction of cells to undergo apoptosis, as demonstrated by DNA fragmentation and quantified by DNA analysis on FACS, yet the majority of the cells died in a necrotic fashion. C3H/HeJ Lpsd macrophages were resistant to LPS-induced cell death and less sensitive to C2 ceramide-evoked cytotoxicity, when compared with Lpsn macrophages. C2 ceramide plus IFN-γ failed to activate release of nitric oxide (NO·), whereas LPS-induced cell death, but not C2-induced cytotoxicity, was blocked by an inhibitor of inducible NO· synthase (iNOS), NG-monomethyl-l-arginine. Macrophages from IFN regulatory factor-1 (−/−) mice shown previously to respond marginally to LPS plus IFN-γ to express iNOS mRNA and NO·, were refractory to LPS plus IFN-γ-induced cytotoxicity and apoptosis. These data suggest that although LPS may mimic certain ceramide effects, signal transduction events that lead to cytotoxicity, as well as the downstream mediators, diverge.

Ceramide activates NFkappaB by inducing the processing of p105

The role of ceramide as a second messenger in tumor necrosis factor (TNF)-mediated signal transduction has been much debated. It is supported by recent reports describing an expanding number of potential targets for this lipid, but is opposed by those describing how ceramide is not necessary for many TNF-mediated cellular events. In this paper, we directly compare the effects of the cell-permeable ceramide analogue, N-acetylsphingosine (C2-ceramide), with TNF, on NFkappaB function, a transcription factor whose activation is central to many TNF-mediated effects. We describe how C2-ceramide failed to drive kappaB-linked chloramphenicol acetyltransferase gene expression in either HL60 promyelocytic or Jurkat T lymphoma cells. Furthermore, it had no effect on TNF-mediated transcription of this reporter gene. However, electrophoretic mobility shift analysis following cell stimulation with this ceramide analogue revealed a dose-responsive activation of NFkappaB, which was not apparent following cell treatment with the inactive dihydro form. Activated complexes from treated cells were shown to contain predominantly the p50 subunit, in contrast to complexes from TNF-treated cells, where both p50 and p65/RelA subunits were present. The specific activation of p50 homodimeric complexes by C2-ceramide, which are known to lack trans-activating activity, was strongly suggested from these data. Further investigations revealed that C2-ceramide had only a marginal effect on IkappaBalpha degradation but strongly promoted the processing of p105 to its p50 product as revealed by immunoblot analysis. The increase in p50 arising from the processing of its p105 precursor was further established from p105/p50 ratios obtained by scanning densitometric analysis of bands from immunoblots. TNF, on the other hand, stimulated both IkappaBalpha degradation and p105 processing, in accordance with previous findings. Furthermore, the effect of TNF on NFkappaB activation was rapid, whereas C2-ceramide required an optimal treatment time of 1 h. Interestingly, TNF was found to increase ceramide in cells but only after a 1-h contact time. Our data therefore suggest that ceramide promotes the activation of NFkappaB complexes that lack transactivating activity by enhanced processing of p105.

Simultaneous quantitation of ceramides and 1,2-diacylglycerol in tissues by Iatroscan thin-layer chromatography-flame-ionization detection

Ceramides and 1,2-diacylglycerol have been demonstrated in intracellular signaling pathways. A method of simultaneous mass determination of ceramides and 1,2-diacylglycerol in tissues was developed using the latroscan which combines thin layer chromatography and flame ionization detection (TLC/FID) techniques. Because of relatively low amounts of these components in tissues, the fraction of nonpolar lipids, which included ceramides and glycerides, was eluted with chloroform/acetone mixture (3:1, vol/vol) through a silicic acid column to eliminate the polar phospholipids. Development of Chromarods was carried out using three solvent systems in a four-step development technique. The relationship of the peak area ratio to weight ratio compared with cholesteryl acetate added as an internal standard was linear. The amount of ceramides increased with incubation of rat heart homogenate and human erythrocyte membranes in the presence of sphingomyelinase (E.C. 3.1.4.12). The latroscan TLC/FID system provided a quick and reliable assessment of ceramides and 1,2-diacylglycerol.

Involvement of different transduction pathways in NF-kappa B activation by several inducers

Double-stimulation was used to demonstrate that, in a T lymphocytic cell line (CEM), phorbol myristate acetate (PMA) rapidly induced NF-kappa B through a signaling pathway which did not involve reactive oxygen species (ROS) and was different from the activation triggered by either H2O2 or tumor necrosis factor-alpha (TNF-alpha). Since these latter compounds were known to activate NF-kappa B translocation in a redox-sensitive way, we have demonstrated that NF-kappa B activation by PMA was resistant to antioxidant N-acetyl-L-cysteine (NAC) and sensitive to kinase inhibitors staurosporine and H7 while activation by H2O2 or TNF-alpha were not.

Selective involvement of ceramide in cytokine-induced apoptosis. Ceramide inhibits phorbol ester activation of nuclear factor kappaB

Among its diverse biologic effects, the cytokine tumor necrosis factor alpha causes the rapid nuclear translocation of the transcription factor, nuclear factor kappaB (NF-kappaB). The p55 tumor necrosis factor (TNF) receptor shares with the related APO-1/Fas antigen the ability to initiate apoptosis. We investigated the role of the sphingolipid mediator ceramide in the cytokine-induced signaling mechanisms leading to NF-kappaB activation and cell death. Several lines of evidence presented here suggest that ceramide generated in response to TNFalpha or Fas activation is not involved in NF-kappaB activation. (i) Cell-permeable ceramides and exogenous sphingomyelinase failed to induce either nuclear translocation of NF-kappaB or degradation of its cytosolic inhibitor, I-kappaB, in Jurkat T cells. (ii) Ceramide treatment of cells inhibited phorbol ester-induced activation of NF-kappaB. (iii) TNFalpha potently activated NF-kappaB in a cell line deficient in acid sphingomyelinase. (iv) TNFalpha activated NF-kappaB within minutes without altering ceramide levels. (v) Treatment of Jurkat cells with cross-linking antibodies to APO-1/Fas induced large scale increases in ceramide and apoptosis without affecting NF-kappaB. (vi) Ceramide generation in response to Fas activation was inhibited by N-acetyltyrosinylvalinylalanylaspartyl chloromethyl ketone, a peptide inhibitor of interleukin-1beta-converting enzyme-like proteases, whereas TNFalpha-induced NF-kappaB activation was unaffected by the inhibitor. These results show that ceramide accumulation belongs selectively to the apoptotic pathway(s) induced by cytokines, and, if anything, ceramide may participate in negative feedback regulation of NF-kappaB.

Anti-immunoglobulin-induced apoptosis in WEHI 231 cells involves the slow formation of ceramide from sphingomyelin and is blocked by bcl-XL

Prolonged (>24 h) exposure to anti-IgM (an antigen surrogate that induces membrane cross-linking and apoptosis) induced a 3-fold increase in the mass of endogenous ceramide measured by 32P labeling by diacylglycerol kinase and a 4-fold increase in ceramide as measured by metabolic labeling with [3H]palmitate in a B-lymphocyte cell line, WEHI 231. This correlated with the induction of apoptosis. Shorter exposure times to anti-IgM (up to 8 h) failed to elicit apoptosis and did not elicit increased ceramide formation. After 8 h, apoptosis occurs concomitantly with ceramide formation over the next 40 h. Further, we showed that exogenous ceramide mimicked anti-IgM-induced apoptosis and that apoptosis was potentiated in serum-free media. Treatment of cells with an inhibitor of ceramide catabolism, N-oleoylethanolamine, increased both ceramide formation and apoptosis and accelerated apoptosis induced by anti-IgM. To examine further how ceramide metabolism is involved in apoptosis, we derived cell lines from a small population of cells resistant to N-oleoylethanolamine. These cell lines were selected based on an altered ceramide metabolic pathway, were resistant to apoptosis induced by anti-IgM, and showed no significant increase in ceramide when challenged with anti-IgM. The basis of this resistance was shown to be the failure to activate neutral sphingomyelinase activity following 24-h treatment with anti-IgM, in contrast to the 2-fold increase in neutral sphingomyelinase activity observed in wild type cells. We have shown previously that transfection of WEHI cells with bcl-xL conferred resistance to anti-IgM-induced apoptosis, whereas transfection with bcl-2 did not (Gottschalk, A., Boise, L., Thompson, C., and Quintans, J. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 7350-7354). In this study, these bcl-xL transfectants also displayed increased resistance to exogenous N-acetylsphingosine (C2-ceramide) or N-hexanoylsphingosine (C6-ceramide). However, when challenged with anti-IgM the bcl-xL transfectants produced levels of ceramide similar to wild type cells, suggesting that ceramide formation is upstream of bcl-xL and that it is a major determinant of B-cell death.

Alteration of the sphingomyelin/ceramide pathway is associated with resistance of human breast carcinoma MCF7 cells to tumor necrosis factor-alpha-mediated cytotoxicity

The interference of tumor necrosis factor-alpha (TNF) signaling processes with the acquisition of tumor resistance to TNF was investigated using the TNF-sensitive human breast carcinoma MCF7 cell line and its established TNF-resistant variant (R-A1). The resistance of R-A1 cells to TNF correlated with a low level of p55 TNF receptor expression and an absence of TNF signaling through TNF receptors. Stable transfection of wild-type p55 receptor in R-A1 resulted in enhancement of p55 expression and in partial restoration of TNF signaling, including nuclear factor-kappaB (NF-kappaB) activation. However, the transfected cells remained resistant to TNF-induced apoptosis. Northern blot analysis revealed a comparable induction of manganous superoxide dismutase and A20 mRNA expression in p55-transfected cells and in sensitive MCF7 cells, making it unlikely that these genes are involved in the resistance to TNF-mediated cytotoxicity. While TNF significantly stimulated both neutral and acidic sphingomyelinase (SMase) activities with concomitant sphingomyelin (SM) hydrolysis and ceramide generation in MCF7, it failed to trigger these events in TNF-resistant p55-transfected cells. In addition, the basal SM content was significantly higher in sensitive MCF7 as compared to the resistant counterparts. Furthermore, the TNF-resistant cells tested could be induced to undergo cell death after exposure to exogenous SMase or cell-permeable C6-ceramide. This study also shows that TNF failed to induce arachidonic acid release in p55-transfected resistant cells, suggesting that an alteration of phospholipase A2 activation may be associated with MCF7 cell resistance to TNF. Our findings strongly suggest a role of ceramide in the mechanism of cell resistance to TNF-mediated cell death and may be relevant in elucidating the biochemical nature of intracellular messengers leading to such resistance.

Activation of the transcription factor NF-kappaB in lipopolysaccharide-stimulated U937 cells

During the course of serious bacterial infections, lipopolysaccharide (LPS) interacts with monocyte/macrophage receptors, resulting in the generation of inflammatory cytokines. Transcription factor NF-kappaB is crucial in activating the transcription of genes encoding proinflammatory cytokines. In this paper, we demonstrate that the activation of NF-kappaB by LPS in a promonocytic cell line (U937) followed a rather slow kinetics, depending on the rate of IkappaB-alpha inhibitor hydrolysis. No degradation of p105 and p100 inhibitors was observed under these conditions. The transduction pathway leading to NF-kappaB activation in U937 cells involved the intracellular generation of reactive oxygen species (ROS), as demonstrated by the concomitant inhibitory effects of antioxidants on NF-kappaB activation and the emission of a fluorescent probe reacting intracellularly with hydrogen peroxide. This ROS pathway was also characterized by the use of other inhibitors. This finding indicates that phospholipase A2 and 5-lipoxygenase are also involved. However, the NF-kappaB activation pathway involving the acidic sphingomyelinase of the endolysosomial membrane did not seem to participate in the LPS-induced NF-kappaB activation in U937 cells.

Cytokine response modifier A (CrmA) inhibits ceramide formation in response to tumor necrosis factor (TNF)-alpha: CrmA and Bcl-2 target distinct components in the apoptotic pathway

Proteases are now firmly established as major regulators of the "execution" phase of apoptosis. Here, we examine the role of proteases and their relationship to ceramide, a proposed mediator of apoptosis, in the tumor necrosis factor-alpha (TNF-alpha)-induced pathway of cell death. Ceramide induced activation of prICE, the protease that cleaves the death substrate poly(ADP-ribose) polymerase. Bcl-2 inhibited ceramide-induced death, but not ceramide generation. In contrast, Cytokine response modifier A (CrmA), a potent inhibitor of Interleukin-1 beta converting enzyme and related proteases, inhibited ceramide generation and prevented TNF-alpha-induced death. Exogenous ceramide could overcome the CrmA block to cell death, but not the Bcl-2 block. CrmA, however, did not inhibit the activation of nuclear factor (NF)-kappa B by TNF-alpha, demonstrating that other signaling functions of TNF-alpha remain intact and that ceramide does not play a role in the activation of NF-kappa B. These studies support a distinct role for proteases in the signaling/activation phase of apoptosis acting upstream of ceramide formation.

Ceramide is not a signal for tumor necrosis factor-induced gene expression but does cause programmed cell death in human vascular endothelial cells

Tumor necrosis factor (TNF) activates transcription of endothelial leukocyte adhesion molecule-1 (CD62E) in endothelial cells (ECs) through the binding to the gene promoter of the p50/p65 heterodimeric form of nuclear factor-kappa B (NF-kappa B) and of the N-terminal phosphorylated form of the ATF2/c-Jun transcription factor, which is phosphorylated by Jun N-terminal kinase (JNK). However, the intracellular signaling pathways that activate endothelial NF-kappa B and JNK in TNF-induced responses are unknown. In this study we have examined the role of a recently described TNF signaling pathway involving sphingomyelin activation to generate ceramide, a potential intracellular mediator. We find that concentrations of TNF that strongly activate NF-kappa B and JNK within 15 minutes do not produce either a measurable decline in sphingomyelin or a measurable generation of ceramide in cultured human umbilical vein ECs at any time examined. Stimulation of ECs with purified sphingomyelinase (SMase) enzyme causes a rapid 60% to 80% decrease in cellular sphingomyelin content and a large increase in ceramide. However, SMase treatment only minimally activates NF-kappa B, achieving levels that are insufficient to initiate gene transcription. Extracellular SMase does not have access to intracellular sphingomyelin, but treatment of ECs with membrane-permeant ceramide analogues still completely fails to activate NF-kappa B and only activates JNK at late times. Neither SMase nor ceramide analogues induce gene transcription or surface expression of endothelial leukocyte adhesion molecules that are readily induced by TNF. Strikingly, low concentrations of membrane-permeant ceramide cause programmed cell death in ECs, a finding not observed at any concentrations of TNF tested. We conclude that ceramide is not an important second messenger for TNF signaling of gene transcription in ECs but may be a second messenger for cell death in response to as-yet-unidentified signals.

Stimulation of the ceramide pathway partially mimics lipopolysaccharide-induced responses in murine peritoneal macrophages

Recent studies have suggested that lipolysaccharide (LPS) stimulates cells by mimicking the second-messenger function of ceramide, a lipid generated in the cell by the action of sphingomyelinase (SMase). To examine this possibility further, we compared the abilities of LPS, SMase, and/or ceramide analogs to induce cytokine secretion, modulate gene expression, and induce endotoxin tolerance in macrophages. SMase and LPS induced secretion of tumor necrosis factor alpha (TNF-alpha) to comparable degrees; however, unlike LPS, SMase failed to stimulate detectable interferon activity. Cell-permeable analogs of ceramide induced the expression of many LPS-inducible genes; however, the expression of interferon-inducible protein 10 (IP-10) and interferon consensus sequence-binding protein (ICSBP) mRNAs was significantly lower than that induced by LPS. Both SMase-induced TNF-alpha secretion and LPS-induced TNF-alpha secretion were inhibited by pretreatment with a serine/threonine phosphatase inhibitor, calyculin A. Macrophages preexposed in vitro to LPS to induce a well-characterized state of endotoxin tolerance secreted little or no TNF-alpha upon secondary challenge with either LPS or SMase, whereas macrophages preexposed to SMase secreted high levels of TNF-alpha upon secondary stimulation with LPS or SMase. Collectively, these results suggest that ceramide activates a subset of LPS-induced signaling pathways in murine peritoneal exudate macrophages.

Ceramide signalling and the immune response

Ceramide, produced through either the induction of SM hydrolysis or synthesized de novo transduces signals mediating differentiation, growth, growth arrest, apoptosis, cytokine biosynthesis and secretion, and a variety of other cellular functions. A generalized ceramide signal transduction scheme is shown in Fig. 2 in which ceramide is generated through the activation of distinct SMases residing in separate subcellular compartments in response to specific stimuli. Clearly, specificity of cellular responses to ceramide depends upon many factors which include the nature of the stimulus, co-stimulatory signals and the cell type involved. Ceramide derived from neutral SMase activation is thought to be involved in modulating CAPK and MAP kinases, PLA2 (arachidonic acid mobilization), and CAPP while ceramide generated through acid SMase activation appears to be primarily involved in NF-kappa B activation. While there is no apparent cross-talk between these two ceramide-mediated signalling pathways, there is likely to be significant cross-talk between ceramide signalling and other signal transduction pathways (e.g., the PKC and MAP kinase pathways). Other downstream targets for ceramide action include Cox, IL-6 and IL-2 gene expression, PKC zeta, Vav, Rb, c-Myc, c-Fos, c-Jun and other transcriptional regulators. Many, if not all, of these ceramide-mediated signalling events have been identified in the various cells comprising the immune system and are integral to the optimal functioning of the immune system. Although the role of the SM pathway and the generation of ceramide in T and B lymphocytes have only recently been recognized, it is clear from these studies that signal transduction through SM and ceramide can strongly affect the immune response, either directly through cell signalling events, or indirectly through cytokines produced by other cells as the result of signalling through the SM pathway. An overview of the signalling mechanisms coupling ceramide to the modulation of the immune response is depicted in Fig. 3 and shows how ceramide may play pivotal roles in regulating a number of complex processes. The SM pathway represents a potentially valuable focal point for therapeutic control of immune responses, perhaps for either enhancement of the activity of T cells in the elimination of tumors, or the down-regulation of lymphocyte function in instances of autoimmune disease. The recent explosion of knowledge regarding ceramide signalling notwithstanding, a number of critical questions need to be answered before a comprehensive, mechanistic understanding can be formulated relative to the incredibly varied effects of ceramide on cell function. For example, (i) how is a structurally simple molecule like ceramide able to mediate so many different, and sometimes paradoxical, physiological responses ranging from cell proliferation and differentiation to inhibition of cell growth and apoptosis, (ii) what are the molecular identities and modes of activation of the various SMase isoforms, (iii) what determines the distribution of the unique isoforms of SMase in cells of different lineages or at different stages of differentiation, (iv) what is the relative contribution of ceramide generated through SM hydrolysis versus de novo synthesis, and (v) by what means does ceramide interact with specific intracellular targets? Although a number of ceramide-activatable kinases, phosphatases, and their protein substrates have been identified, a more extensive search for additional cellular targets will be indispensable in determining the phosphorylation cascades linking the activation of the SM pathway to the regulation of nuclear events. Clearly, cross-talk between ceramide-induced signal transduction cascades and other signalling pathways adds to the inherent difficulty in distinguishing the specific effects of complex, intertwining signalling pathways.

Interleukin-1 signal transduction

Interleukin-1 (IL-1) is primarily an inflammatory cytokine, although it is capable of mediating a wide variety of effects on many different cell types. Nearly every known signal transduction pathway has been reported to be activated in response to IL-1. However, the significance of many of these signaling events is unclear, due to the use of different and sometimes unique cell lines in studying IL-1-initiated signal transduction. Complicating matters further is the lack of association in many studies between identified IL-1-induced signals and subsequent biological responses. In this article, we review what is known about IL-1 receptor signaling and, whenever possible, correlate signaling events to biological responses.

Endothelial cell inflammatory responses to tumor necrosis factor alpha. Ceramide-dependent and -independent mitogen-activated protein kinase cascades

Ceramide generation by stimulated sphingomyelinase activity has been implicated in tumor necrosis factor alpha (TNF) signaling of apoptosis and differentiation. We examined the role of ceramide in a major action of TNF: the initiation of inflammatory events. Sphingomyelinase C at high levels induced inflammatory protein expression in endothelial cells resulting in leukocyte adhesion, but the pattern of induction of adhesion molecules (E-selectin, ICAM-1, VCAM-1) and cytokines (interleukins 6 and 8) differed from that induced by TNF. TNF induced only a small increase in ceramide: using lower doses of sphingomyelinase to mimic this we found that small amounts of ceramide did not induce protein expression, but still rapidly activated Raf-1, mitogen-activated protein/extracellular regulated kinase (ERK) kinase (MEK) and ERKs. TNF additionally caused rapid p38 and JNK-1 mitogen-activated protein kinase activation and efficient NF-kappaB translocation, which could not be achieved even by high levels of ceramide. Thus activation of the ERK cascade alone is an incomplete endothelial cell stimulus, and the TNF receptor generates at least two signals: Raf-1 activation, which could be ceramide-dependent; and ceramide-independent efficient NF-kappaB translocation and activation of p38 and JNK-1 mitogen-activated kinases.

Interleukin-1 beta-induced ceramide and diacylglycerol generation may lead to activation of the c-Jun NH2-terminal kinase and the transcription factor ATF2 in the insulin-producing cell line RINm5F

The aim of this investigation was to study the putative involvement of lipid second messengers, protein kinases, and transcription factors in interleukin-1 beta (IL-1beta)-induced signal transduction in insulin-producing cells. For this purpose, insulin-producing RINm5F cells were exposed to IL-1beta (25 units/ml), and the ceramide, ceramide 1-phosphate, sphingomyelin, diacylglycerol, and phosphatidic acid contents of the cells were subsequently determined. It was found that IL-1beta induced a transient increase (2-5 min) in ceramide and diacylglycerol, which was not paralleled by an increase in ceramide 1-phosphate and phosphatidic acid. A rapid decrease in the sphingomyelin content of the cells was, however, observed. The cell-permeable ceramide analogue N-acetylsphingosine and the phorbol ester phorbol 12-myristate 13-acetate (PMA) both induced the phosphorylation and increased the activities of the protein kinase JNK1 and the transcription factor ATF2. These effects were, however, not as pronounced as those induced by IL-1beta. The DNA binding activity of transcription factors in nuclear extracts was determined using the electrophoretic mobility shift assay method. Transcription factor binding to the ATF/cAMP-responsive element consensus sequence was increased 4-5-fold by acetylsphingosine, PMA, or IL-1beta, whereas binding to the CCAAT/enhancer-binding protein and AP-1 elements was found to be only slightly stimulated by these three agents. Binding to the NF-kappaB element was strongly induced by IL-1beta, but not by acetylsphingosine or PMA. Finally, acetylsphingosine and PMA did not mimic the nitric oxide-inducing effects of IL-1beta. It is concluded that IL-1beta-stimulated formation of ceramide and diacylglycerol may contribute to JNK1 and ATF2 transcription factor activation, which may be a necessary (but not sufficient) step in beta-cell nitric-oxide synthase induction.

Characterization of the sphingomyelin content of isolated pancreatic islets. Evaluation of the role of sphingomyelin hydrolysis in the action of interleukin-1 to induce islet overproduction of nitric oxide

Inflammatory cytokines may participate in the destruction of pancreatic islets during the pathogenesis of insulin-dependent diabetes mellitus, and the cytokine interleukin-1 (IL-1) strongly inhibits insulin secretion from rat pancreatic islets by a process which involves induction of expression of the inducible isoform of nitric oxide synthase and the overproduction of nitric oxide. The signaling events between IL-1 receptor occupancy and induction of nitric oxide synthase in rat islets involve activation of the transcriptional activator NFkappa B. Because sphingomyelin hydrolysis has been implicated as a signaling process both in NFkappa B activation and in IL-1 action in some cells, we have examined the potential involvement of sphingomyelin hydrolysis in the induction of islet nitric oxide overproduction by IL-1. Rat islet sphingomyelin pools were radiolabeled with [3H]choline, and sphingomyelin was then isolated by normal phase HPLC. Electrospray ionization-mass spectrometric analysis revealed islet sphingomyelin consists of at least 4 distinct molecular species, and the most abundant of them contained sphingosine as the long chain base and a residue of palmitic acid as the fatty acid substituent. Molecular species containing residues of stearic acid and arachidic acid were also observed. Neither interleukin-1 nor tumor necrosis factor-alpha was found to induce hydrolysis of islet sphingomyelin species, and neither an exogenous, cell-permeant ceramide species (N-acetyl-D-sphingosine) nor exogenous sphingomyelinase mimicked or potentiated the effect of IL-1 to increase rat islet nitric oxide generation, as reflected by nitrite production. Similar findings were obtained with RINm5F insulinoma cells and with mouse pancreatic islets. These findings provide the first information on the molecular species of sphingomyelin in pancreatic islets and suggest that sphingomyelin hydrolysis is not involved in the signaling pathway whereby IL-1 induces the overproduction of nitric oxide by pancreatic islets.

A simple method for screening for Farber disease on cultured skin fibroblasts

Farber disease is an inborn lysosomal storage disorder characterized by accumulation of ceramide in the patient's tissues due to the deficient activity of acid ceramidase. Currently, confirmation of the diagnosis is performed in an extremely limited number of laboratories. We therefore developed a procedure which does not require any particular sphingolipid substrate and is based on the quantitation of ceramide levels in cultured skin fibroblasts. In the method we devised, the ceramide present in cellular lipid extracts subjected to mild alkaline hydrolysis was quantified using the commercially available diacylglycerol kinase kit. We show that both primary cultures of skin fibroblasts and SV40-transformed fibroblasts derived from a series of patients with Farber disease exhibit ceramide excess as compared to their normal counterparts (2345-17 153 pmol/mg cell protein in Farber cells vs. 432-1298 pmol/mg cell protein in controls). Use of this simple method should greatly facilitate the biochemical diagnosis of Farber disease.

Interaction of ceramides, sphingosine, and sphingosine 1-phosphate in regulating DNA synthesis and phospholipase D activity

C2- and C6-ceramides (N-acetylsphingosine and N-hexanoylsphingosine, respectively) abolished the stimulation of DNA synthesis by sphingosine 1-phosphate in rat fibroblasts. This inhibition by ceramide was partially prevented by insulin. C2-ceramide did not alter the stimulation of DNA synthesis by insulin and decreased the sphingosine-induced stimulation by only 16%. The ceramides did not significantly modify the actions of sphingosine or sphingosine 1-phosphate in decreasing cAMP concentrations. C2- and C6-ceramides blocked the activation of phospholipase D by sphingosine 1-phosphate, and this inhibition was not affected by insulin. Okadaic acid decreased the activation of phospholipase D by sphingosine 1-phosphate and did not reverse the inhibitory effect of C2-ceramide on this activation. Therefore, this effect of C2-ceramide is unlikely to involve the stimulation of phosphoprotein phosphatase activity. Sphingosine did not activate phospholipase D activity significantly after 10 min. C2-ceramide stimulated the conversion of exogenous [3H]sphingosine 1-phosphate to sphingosine and ceramide in fibroblasts. Ceramides can inhibit some effects of sphingosine 1-phosphate by stimulating its degradation via a phosphohydrolase that also hydrolyzes phosphatidate. Furthermore, C2- and C6-ceramides stimulated ceramide production from endogenous lipids, and this could propagate the intracellular signal. This work demonstrates that controlling the production of ceramide versus sphingosine and sphingosine 1-phosphate after sphingomyelinase activation could have profound effects on signal transduction.

Regulation of cytochrome P450 2C11 (CYP2C11) gene expression by interleukin-1, sphingomyelin hydrolysis, and ceramides in rat hepatocytes

Interleukin-1 triggers the down-regulation of several hepatic cytochrome P450 gene products, but the cellular signaling pathways involved are not known. We have examined the role of sphingomyelin hydrolysis to ceramide in the suppression of CYP2C11, a major constitutive form of cytochrome P450, by interleukin-1. Treatment of rat hepatocytes cultured on matrigel with interleukin-1 beta caused a rapid turnover of sphingomyelin and an increase in cellular ceramide, with no change in cellular phosphatidylcholine. The ceramide was composed mainly of a D-erythro-sphingosine backbone, suggesting that it was derived from sphingolipid hydrolysis rather than from increased de novo synthesis. Treatment of the cells with either N-acetyl-D-erythro-sphingosine (C2-ceramide) or bacterial sphingomyelinase suppressed the expression of CYP2C11 and induced the expression of the interleukin-1-responsive alpha 1-acid glycoprotein mRNA. In contrast, the acute-phase gene beta-fibrinogen, which is induced by interleukin-6 but not by interleukin-1, did not respond to C2-ceramide. N-Acetyl-D-erythro-sphinganine mimicked the effect of C2-ceramide on CYP2C11, but not on alpha 1-acid glycoprotein expression. These results are consistent with a role for ceramide or a related sphingolipid in mediating the down-regulation of CYP2C11, the induction of alpha 1-acid glycoprotein, and perhaps other cellular effects of interleukin-1 in hepatocytes.

Low temperatures and hypertonicity do not block cytokine-induced stimulation of the sphingomyelin pathway but inhibit nuclear factor-kappa B activation

In order to better understand the significance of tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta)-receptor internalization in the sphingomyelin pathway signal transduction, we investigated receptor signaling under conditions in which receptor internalization is blocked. We demonstrate that human recombinant TNF-alpha and IL-1 beta both induced sphingomyelin and phosphatidylcholine hydrolysis at either 4, 14, or 37 degrees C in human skin fibroblasts and U937 monocytic cells. Cytokine-induced sphingomyelin degradation also occurred when endocytosis was inhibited by incubating the cells in hypertonic medium. While internalization was not required for the production of ceramide, activation of the transcription factor NF-kappa B was strongly reduced when cells were stimulated with TNF at low temperature or in hypertonic medium. Under these conditions, activation of NF-kappa B by the cell-permeant C2- ceramide (N-acetylsphingosine), by exogenous sphingomyelinase or by phorbol myristate acetate was also inhibited. These results suggest that low temperature and hypertonicity, two inhibitors of receptor internalization: (i) do not affect the TNF-alpha- or IL-1 beta-induced sphingomyelin hydrolysis, but (ii) do inhibit a step distal to ceramide of the intracellular signaling pathway leading to NF-kappa B activation.

Ceramide activates the stress-activated protein kinases

Tumor necrosis factor alpha (TNF alpha) activates the stress-activated protein kinases (SAPKs, also known as Jun nuclear kinases or JNKs) resulting in the stimulation of AP-1-dependent gene transcription and induces the translocation of NF kappa B to the nucleus resulting in the stimulation of NF kappa B-dependent gene transcription. A potential second messenger for these signaling pathways is ceramide, which is generated when TNF alpha activates sphingomyelinases. We show that treatment of HL-60 human promyelocytic cells with exogenous sphingomyelinase leads to rapid stimulation of JNK/SAPK activity, an effect not mimicked by treatment with phospholipase A2, C, or D. Further, JNK/SAPK activity is stimulated 2.7- and 2.8-fold, respectively, in cells exposed to C2-ceramide (5 microM) or TNF alpha (10 ng/ml). The prolonged stimulation of this kinase activity by C2-ceramide is similar to that previously reported for TNF alpha. In contrast, the related mitogen-activated protein kinases ERK1 and ERK2 are weakly stimulated following TNF alpha treatment (1.5-fold) and are inhibited by C2-ceramide treatment. TNF alpha also potently stimulates NF-kappa B DNA binding activity and transcriptional activity, but these effects are not mimicked by addition of C2-ceramide or sphingomyelinase to intact cells. Furthermore, TNF alpha, sphingomyelinase, and C2-ceramide induce c-jun, a gene that is stimulated by the ATF-2 and c-Jun transcription factors. These data suggest that ceramide may act as a second messenger for a subset of TNF alpha's biochemical and biological effects.

Ceramide as a modulator of endocytosis

The effects of ceramide analogs on the uptake of markers for fluid-phase (horseradish peroxidase, HRP) and receptor-mediated (low density lipoprotein, LDL) endocytosis were studied in Chinese hamster fibroblasts. N-Hexanoyl-D-erythro-sphingosine (C6-Cer) decreased the uptake of HRP in a dose-dependent manner. Internalization was inhibited > 40% with 25 microM C6-Cer, relative to controls, and was apparent within 5 min. Internalization of HRP was also inhibited by other Cer analogs and by treatment with agents that raise levels of endogenous Cer (sphingomyelinase or the glycosphingolipid synthesis inhibitor, 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP)), but not by N-hexanoyl-D-erythrosphinganine (C6-dihydro-Cer) or sphingosine. Internalization of LDL was also inhibited by C6-Cer in a concentration-dependent manner, but was less pronounced than the effect on HRP internalization (10% versus 40% inhibition with 25 microM C6-Cer), suggesting that ceramide might affect fluid-phase and receptor-mediated endocytosis to different extents. C6-Cer also slowed HRP and LDL transport from endosomes to lysosomes as studied by analysis of endocytic vesicles on Percoll density gradients and induced a redistribution of endocytic organelles as determined by fluorescence microscopy of intact cells using appropriate markers. This resulted in decreased degradation of 125I-labeled LDL in the presence of C6-Cer. These results suggest that endogenous ceramide may modulate endocytosis.